High efficiency light valve projection system

ABSTRACT

A light valve such as an active matrix LCD between crossed polarizers, utilizing, for instance, individual transistors to control each &#34;pixel area&#34; of the LCD and storage elements to store video signal data for each pixel, with optically shielded &#34;dead spaces&#34; between pixels to eliminate electric field cross-talk and non-information-bearing light bleed-through, is illuminated with a bright independent light source which creates a video image projected via specialized projection optics onto an internal or external screen without distortions, regardless of the angle of projection onto the screen. Use of heat sinks, IR reflective coatings, heat absorbing optics, optional fluid and a thermistor controlled pixel transistor bias voltage injection servo circuit stabilizes image performance, maintaining accurate color and contrast levels as the LCD changes temperature. In one embodiment of the invention, use of a multi-color LCD with a stepped cavity, producing different thicknesses of LCD for the different wavelengths that pass through it, allows a linear correspondence between the wavelengths passing through the LCD to produce true black, high contrast and CRT-like color rendition. A dichroic mirror arrangement is used to overlap differently colored pixels in the projected image. Use of striped mirrors duplicate pixels, where necessary, eliminating spaces between pixels, creating a continuous image with no apparent stripes or dots. A special venetian-blind type of screen is also disclosed and methods for using the system to view three-dimensional video are also explained.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.08/384,720, filed Feb. 6, 1995, now abandoned and a divisionalapplication of Ser. No. 08/223,479 filed Apr. 4, 1994 now abandoned,which is a continuation-in-part of U.S. application Ser. No. 659,596,entitled A HIGH EFFICIENCY LIGHT VALVE PROJECTION SYSTEM, filed Feb. 21,1991, now U.S. Pat. No. 5,300,942 which is a continuation-in-part ofU.S. application Ser. No. 07/290,040, entitled "AN ACTIVE MATRIX LCDIMAGE PROJECTION SYSTEM," filed Dec. 23, 1988, now U.S. Pat. No.5,012,274 which is a continuation-in-part of U.S. application Ser. No.07/140,233, entitled "AN IMPROVED VIDEO DISPLAY SYSTEM," filed Dec. 31,1987 abandoned.

FIELD OF THE INVENTION

The present invention relates generally to video and data displaydevices and more particularly to an improved video display systememploying light valves such as an active matrix LCD in conjunction withnovel projection optics.

BACKGROUND OF THE INVENTION

The mainstay of electronic imaging, since its beginning, has been thecathode ray tube (CRT) or kinescope. Although CRT technology hasprogressed over the years, several major drawbacks remain. Picture sizeis still limited, making group viewing difficult. CRT picture tubeslarger than about 30" (measured diagonally) become impractical becauseof size, weight, expense and danger of implosion because of the highvacuum used. To achieve high brightness they use dangerously highvoltages and may produce health hazards from x-rays and electromagneticfields.

Image quality of CRT-based video displays may be degraded by colordistortion, image shape distortions, color impurity from the influenceof the earth's magnetic field, and color misconvergence. In addition,CRT displays are subject, particularly when viewed at close range, tovisual artifacts such as scanning lines and discrete phosphor dots orstripes, which are inherent in such TV displays. These visual artifactsprovide a poorer image quality than images in movie theaters.

Research has been continuing on for many years to develop other types oflight emissive displays which would overcome some of these drawbacks.Plasma, electroluminescent (EL) and cold cathode phosphor displays areamong the most promising candidates, although they have not provedthemselves to be practical. Furthermore, it is highly questionablewhether these other emissive displays, if and when successful, wouldprovide any advances over current CRT brightness or size in practicalapplications. "Pocket Tvs" with a picture size of 2"3" are constructedtoday using liquid crystal displays which are addressed via electronicmultiplexing or active matrix addressing. Creating a large picture fordirect viewing however poses many problems which have heretofore notbeen overcome. Simple multiplexing cannot produce a satisfactory imagebecause of cross-talk. An active matrix relieves the cross-talkproblems, but has so many more production steps and so many switchingand storage elements that must be deposited over a large surface areathat production of large, defect-free active matrix displays for directviewing has not been possible and may never be economically feasible forvery large displays.

Demand for large video imaging systems and for thin profile or "flatscreen" imaging systems, both large and small, has increasedsignificantly in recent years and is expected to increase dramaticallywith the advent of high definition television broadcasts. "Projectiontelevisions" have been developed and commercialized in recent years.Unfortunately, such projection display devices have exacerbated many ofthe problems associated with earlier video display systems and havecreated new problems. Projection televisions are more expensive thanstandard direct-view televisions and are more cumbersome, heavier, andlarger so that portability is impractical. Two types of projectiontelevision systems have become popular: one using three CRTs withprojection lenses and the other using an oil film scanned by an electronbeam.

The CRT-based projection system remains relatively dim, requiring adimly-lit viewing environment and a costly special screen which providesa very limited viewing angle. The three CRTs produce images in theprimary colors, blue, green, and red and are driven with higher anodevoltage than conventional systems to obtain as much brightness out ofthem as possible. The higher anode voltage lowers tube life andincreases the radiation hazards and other problems associated with highvoltage. The three tubes also increase the danger of tube implosion. Thestandard oil-based system, referred to as an Eidophor, has three"scanned oil elements" which have a relatively short life and useexternal light sources. In either system, all three color imagesutilizing three sets of optics must be precisely converged onto theviewing screen, in addition to requiring adjustments of hue, saturation,vertical and horizontal size and linearity, and minimization ofpincushion and barrel distortion. Proper alignment in either system istherefore beyond the abilities of the average person. Proper convergenceis not easily achieved and often requires up to a half hour ofadditional set-up time because of the curvature of the lenses andvariations in the performance of the circuits in either system. If theprojector or screen is moved, the convergence procedure must berepeated.

Experimentation has also been performed on laser systems which scan outan image on a viewing screen in the same way an electron beam scans theimage onto the face of a CRT. The laser systems developed thus far aremuch too large to be portable, very complex to use and maintain,extremely expensive, potentially dangerous and have proven too dim forlarge images.

Many attempts have been made to solve the above-mentioned problems,resulting in experimentation on several novel "light valve" basedsystems. This type of system uses an external light source which cantheoretically be as bright as desired, with a "light valve" to modulatethe light carrying the picture information. The research andexperimentation to develop a workable light valve system has beenprimarily directed to using different optical, electronic, physical andother effects and finding or producing various materials to accomplishthe desired results. The various light-valve system attempts have mainlyutilized crystals (such as quartz, Potassium Di-Hydrogen Phosphate,Lithium Niobate, Barium Strontium Niobate, Yttrium, Aluminum, Garnet andChromium Oxide), liquids (such as Nitro Benzene) or liquid crystals (ofthe smectic or nematic type or a suspension of particles such asiodoquinine sulphate in a liquid carrier) or other similar materials totake advantage of one or more optical effects including electro-opticaleffects, such as creating a rotated plane of polarization or alteringthe index of refraction of the material due to an applied electricfield, magneto-optical effects using an applied magnetic field,electro-striction effects, piezo-optical effects, electrostatic particleorientation, photoconductivity, acousto-optical effects, photochromiceffects and laser-scan-induced secondary electron emission. Except forliquid crystal light valves, such light valves proved impossible tomanufacture economically and with a sufficiently large aperture and haveoften been toxic, dangerous, and inconsistent in production quality.

In all light valves, different areas must be supplied differentinformation or "addressed," so that a different amount of light emergesthrough each area, adding up to a complete picture across the total beamof light. Techniques for addressing different picture elements (or"pixels") of a light valve have included methods for deflecting a laseror electron beam to that area or the use of a tiny criss-cross ofelectrically conductive paths, i.e., a matrix, deposited on or adjacentto the material to be addressed in order to activate that area of thematrix. In scanning beam systems, problems have included outgassing anderosion of material. The electrical matrix system has proved difficultto engineer, requiring deposition with extremely high precision of atransparent material having good conductivity characteristics. Further,such matrices must be driven by extremely fast switching circuits, whichare impractical at the high voltages required to activate a given areaof most materials.

The most frequently used system for addressing small areas is oftenreferred to as electronic multiplexing. Electronic multiplexing workswell with only low voltage-requiring materials such as liquid crystals.With this method, all pixel addresses are x and y coordinates on theconductive grid. To activate a given pixel area a specific amount,different voltages must be applied to the x and y conductors so that,where they meet, they together exceed a threshold voltage and modulatethe area. A major drawback to such multiplexing is cross-talk, wheresurrounding areas are affected by the electric field, causing false datato influence surrounding pixels, reducing contrast and resolution, aswell as color saturation and accuracy. The cross-talk problem increaseswhen resolution increases because liquid crystal materials respondfairly linearly to applied voltage. Since all pixels are interconnectedwithin the same system, all pixels are given partial voltage and are,thus, partially activated when any one pixel is addressed. Non-linearmaterials can be added to the liquid crystal mix, but this still doesn'tallow for more than about 160 lines of resolution before cross-talksignificantly degrades the image.

An "active matrix" light valve in which all pixels from the matrix areselectively disconnected except for those pixels which are addressed atany given time eliminates the cross-talk problem, regardless of thenumber of pixels or lines in a display. Recently, active matrix displayshave been made utilizing transistors, diodes, or an ionizing gas as theswitching element to disconnect the pixels.

Since liquid crystal light valves have very little persistence and onepixel or line of pixels is activated at a time, substantially less lightis projected to the screen to be ultimately viewed since all pixels are"off" most of the time. This characteristic wastes light, produces adimmer image with poorer contrast and generates more heat because of thebrighter source necessary to compensate for the dim image. High refreshrates are impractical because they would require faster switching timesand faster responding material.

Active matrix displays, however, also utilize a storage element, such asa capacitor, connected to each pixel, which allows each pixel to retainthe proper charge, and thus, the proper transmissivity after the pixelhas been addressed and disconnected from the system. Thus, each pixelremains "on" the correct amount all the time. This increases lightthroughput and eliminates flicker.

If high-wattage light sources are used in order to achieve very brightdisplays, heat sensitivity can cause a decrease in contrast and colorfidelity. Absorption of high intensity light by color filters andpolarizers (if used), even if little or no infrared light is present,results in heating of these elements which can also degrade imagequality and may even damage the light valve. Use of fan cooling causesobjectionable noise, especially in quiet environments when source volumeis kept low.

Another inherent problem of light valve projection systems relates tothe fact that each pixel of the frame is surrounded by an opaque borderthat contains addressing circuitry or physical structure. This resultsin visibly discrete pixels and contributes an objectionable "graininess"to the image that become progressively more annoying when viewed atclose distance or on large screens. The problem is amplified if a singlefull-color light valve is used in which the individual red, green, andblue color elements of each pixel are not converged or blended and arevisible to the viewer.

Consequently, projection by means of a small light valve provides themost practical and economical way to produce a large, bright image.Unfortunately, such light valve projectors have, up to the present,exhibited several shortcomings which fall generally into at least fourbroad categories, namely:

1) light valve restrictions;

2) light source limitations;

3) optical system inefficiencies; and

4) screen performance weaknesses.

These problems must be addressed to allow for the successful productionof acceptable quality, practical display systems, capable of largeprojection imagery and display of small or large images from a devicewith a "thin profile."

To address these and other problems associated with prior art videodisplay systems, it is an object of the present invention to provide anadjustable size video image which can be very large, yet possesses highquality and sufficient brightness to be visible from wide viewing angleswithout distortions, in a normally lit room as well as in environmentswith high ambient light.

Furthermore, an object of the invention is to create a video displaysystem which utilizes a light valve such as a specially constructed LCDlight valve, an independent light source with a long life, highbrightness, average luminance, and color temperature, and novel optics,providing for high light efficiency for front or rear projection andwhich operates without excess heat or fan noise.

Another object of the invention is to produce such a system with highresolution and contrast (eliminating the appearance of stripes, pixels,or lines), with highly accurate color rendition (equal to or better thanthat of a CRT).

An additional object of the invention is to produce a display thatreduces eye strain by the elimination of flicker and glare and by thebroadening of color peaks.

A further object of the invention is to produce a small, lightweight,portable system, having a long maintenance-free operating life, which isoperable in conjunction with or without a special screen and can bemass-produced relatively inexpensively.

Yet another object of the invention is to produce a system whichrequires no convergence or other difficult adjustments prior to viewing.

Still another object of the present invention is to produce a systemwith greatly reduced radiation and hazard of tube implosion and operateswith relatively low voltage.

An additional object of the invention is to produce a system which doesnot require a special screen, can be easily projected onto a wall orceiling, and can be viewed comfortably at relatively wide angles.

A further objective of the invention is to produce such a system capableof three-dimensional projection.

Additional objects of the invention include the creation of a systemwhich will overcome drawbacks associated with CRTs in terms of weight,bulk, high voltage, radiation, implosion hazard and convergencedifficulty in 3-CRT projection systems.

Further objects will include increasing image contrast, colorreproducibility, resolution and yield while reducing color pixelvisibility, flicker, heat sensitivity, image artifacts, system coolingnoise and bleed-through of non-image bearing light, while decreasing thecost and complexity of light valve systems.

Additional objects of the invention involve creating a system toovercome and improve upon light source limitations by increasingbrightness efficiency, average luminance and color temperature, whilelengthening bulb life and reducing the weight and bulk of the powersupply.

Yet additional objects of the invention involve creating a system withimproved light collection, decreased light losses due to color selectionand polarization, decreased light valve aperture ratio losses and othernon-image light waste.

Further objects of the invention involve creating a system whichinvolves improving performance by use of particular screen materialswith reduced light absorption, while reducing lenticular-lens-patternimage degradation, off-axis projection distortion and off-axisbrightness fall-off, while reducing the effect of glare and ambientlight to image visibility.

Moreover, it is an object of the invention to create a system whichminimizes and virtually eliminates the wasted space of projectiondistance and enables three-dimensional projection.

Other objects will become evident from the disclosure.

SUMMARY OF THE INVENTION

These and other objects of the invention which will become apparenthereafter are achieved by "A HIGH EFFICIENCY LIGHT VALVE PROJECTIONSYSTEM" employing a light valve, such as a liquid crystal display (LCD)device, for the formation of an image utilizing an "active matrix" forelectronically addressing and activating each of the liquid crystalelements in the matrix. The matrix is "active" in that a separatetransistor or other suitable material is located adjacent to eachpicture element or "pixel" to control each pixel and a storage element,such as a capacitor, which is employed to store the respective pixelvideo signal. The system further comprises a direct projection opticsarrangement which includes a light source for illuminating the lightvalve, optics which collimate light from the source and improve lightthroughput efficiency and quality of the projected image and a lenssystem for projecting and focusing an image from the light valve onto aviewing surface.

An important aspect of one embodiment of the invention is the use of adichroic mirror system to superpose color pixel triads from a single,multicolored LCD to form full-colored pixels with spaces between them.

Another aspect of one embodiment of the invention relates to the fillingof spaces between pixels. These spaces may be filled using a four-mirrorsystem, in which a first striped mirror pair duplicates each pixel andthe image is shifted horizontally into the spaces which previouslyexisted between pixels. A second mirror pair duplicates the newlycreated rows of pixels and shifts the original and the duplicated pixelimages vertically to fill the remaining spaces between pixels.

Other methods are described for the filling of spaces between adjacentpixels through the use of an expanding lens array and a collimating lensor a second collimating lens array to expand and collimate individualimages of the pixels.

The invention will be better understood by the Detailed Description ofthe Preferred Embodiment in conjunction with the appended drawings, ofwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the invention depicting three LCDsprojecting their image onto one common screen;

FIG. 2 is a schematic view of a modified embodiment of the presentinvention in which the images of three LCDs are internally superposedand projected onto a common screen employing one set of projectionoptics;

FIG. 3 is a schematic view of various pixels with reduced spaces betweenthem;

FIG. 4 is a schematic view of a projected image of superposed"full-color pixels";

FIG. 5 is a schematic view of a four mirror system depicting a method offilling in spaces between adjacent pixels;

FIG. 6 is a schematic view depicting the filling of spaces betweenpixels by the first two-mirrors (a "striped-mirror pair") of thefour-mirror system of FIG. 5;

FIG. 7 is an enlarged schematic view of a "striped-mirror pair" of thefour-mirror system of FIG. 5;

FIGS. 8a and 8b are schematic views of lens-system embodiments of thepresent invention;

FIG. 9a is a schematic view of a dichroic mirror system of oneembodiment of the present invention;

FIG. 9b is a schematic view of the embodiment of the dichroic mirrorsystem of FIG. 9a, modified to include an additional light path;

FIG. 10 is a graphical plot of transmitted light intensity over thevisible spectrum through two full-color LCDs, one with a constant LCDcavity thickness contrasted with a "stepped thickness" LCD cavity;

FIG. 11a is graphical plot of transmitted light intensity vs. appliedvoltage for three wavelengths used in two full-color LCDs, with thelefthand part for a constant thickness LCD cavity and the righthand partfor a "stepped thickness" LCD cavity;

FIG. 12 is a magnified schematic view of a "stepped thickness" LCDcavity showing the different thicknesses of LCD through which the red,green, and blue lights traverse;

FIG. 13 is a CIE chromaticity diagram comparing color ranges of a CRTdisplay, a conventional color LCD display with a fixed cavity thicknessand a "stepped thickness" LCD cavity in accordance with the presentinvention;

FIG. 14 is a schematic view of a rear-screen projection system utilizingthe present invention with a venetian-blind type of rear-projectionscreen;

FIG. 15a is a schematic view of color filters on correspondingcolor-pixel areas in a full-color LCD;

FIG. 15b is a schematic view of an alternate arrangement of pixels inwhich three pixels of a color triad are indicated by a triangle;

FIG. 16 is an open perspective view of a sound suppression system whichmay be adapted to the present invention;

FIG. 17 is a schematic diagram of the preferred embodiment of theinvention;

FIG. 18 is a schematic view of an active matrix liquid crystal displaywhich utilizes a gas as a switching element to disconnect pixels fromthe circuit;

FIG. 19 is a schematic view of an embodiment of the electronic imageprojection system in which two light valves are placed together whereone light valve would compensate for defective pixels in the other lightvalve;

FIG. 20 is a schematic view of a projection arrangement utilizing areflective light valve;

FIG. 21 is a schematic view of a single light valve divided into threesections for use in full-color projection;

FIG. 22 is a schematic view of a method of matching the path lengths ofbeams travelling from a light valve to a projection lens utilizing apath length compensation lens;

FIG. 23 is a schematic view of a technique utilizing mirrors tocompensate for path length differences of beams travelling from thelight valve to the projection lens in an embodiment of the presentinvention;

FIG. 24 is an alternate embodiment of the electronic image projectionsystem utilizing a reflective light valve to produce a full-color imageand a MacNeille prism for polarizing and analyzing beams;

FIG. 25 is a schematic view of a section of the electronic imageprojection system in which dichroic mirrors separate a collimated beamof a white light into colored beams of light which pass through a doublelens array creating demagnified collimated beams of colored lightarranged side by side by a second set of dichroic mirrors for use as amulticolored beam to illuminate a full-color light valve;

FIG. 26 a schematic view of an alternate method of producing amulticolored light beam in the electronic image projection system foruse in illuminating a multicolored light valve;

FIG. 27 is a schematic view of an alternate method of producing amulticolored light beam utilizing a hologram to separate a white lightbeam into red, green and blue beams and a second hologram to makeparallel the resulting beams;

FIG. 28 is a schematic view of wedges used in the optical path of aprojector to create three overlapping images of the full-color lightvalve so as to merge red, green and blue pixel color components intofull-color pixels in the image;

FIG. 29 is a schematic view of a four-mirror system in the electronicimage projection system to overlap red, green and blue pixel colorcomponents creating fullcolor pixels;

FIG. 30 is a schematic view of a two-mirror system in an alternateembodiment of the electronic image projection system used to superimposered, green and blue pixel color components creating full-color pixels;

FIG. 31 is a three-mirror system in an alternate embodiment of theelectronic image projection system to superimpose red, green and bluepixel color components creating full-color pixels;

FIG. 32 is a schematic view of the classic method of spatial filteringusing a lens to perform Fourier transformation;

FIG. 33 is a schematic view of an electronically controlled prism forimage displacement to be used with the present invention;

FIG. 34 is a schematic view of pixel holes in a light valve with alenslet before and after the pixel hole for use in analysis ofillumination uniformity in one aspect of the electronic image projectionsystem;

FIG. 35 is a schematic side view of a light valve and lens arrays forfurther analysis of an aspect of the electronic image projection system;

FIG. 36 is a schematic side view of an embodiment of a section of theelectronic image projection system utilizing field lens arrays with alight valve;

FIG. 37 depicts a schematic view of a section of the electronic imageprojection system in which two light sources are used whose beams arecollimated and made continuous by the use of a prism;

FIG. 38 is a schematic view of a section of the electronic imageprojection system in which light from two collimated beams isredistributed by the use of mirrors to produce a single beam with aGaussian-like distribution that would be found in a single beam;

FIG. 39 is a schematic view of a section of the electronic imageprojection system in which a parabolic reflector is used in conjunctionwith a conventional spherical reflector and condenser lens to capturemore light for use in projection;

FIG. 40 is a schematic view of a Galilean telescope which may be used toreduce a collimated beam diameter to a smaller collimated beam;

FIG. 41 is a schematic view of an alternate embodiment of a section ofthe electronic image projection system in which an elliptical mirror isused in conjunction with two collimating lenses to capture and useotherwise lost light;

FIG. 42 is a schematic view of a segment of the electronic imageprojection system in which multiple condenser paths are used to capturemore light from a light source for use in projection;

FIG. 43 is a schematic view of a section of the electronic imageprojection system in which separate light beams are caused to become asingle light beam by bringing the beams to separate foci and using amirror to redirect one of the beams so that the two beams becomecollinear;

FIG. 44 is a schematic view of an embodiment of a section of theelectronic image projection system in which mirrors are used to rotatethe polarization plane of a beam coming from a MacNeille prism to makethe resulting beam parallel with another beam from the MacNeille prism;

FIG. 45 is a schematic view of a section of the electronic imageprojection system in which two collimated beams are made contiguous bythe use of a mirror;

FIG. 46 is a schematic view of an alternate embodiment of a section ofthe electronic image projection system in which a parabolic surface isused to capture and collimate light that misses an elliptical reflectorin a light collection system;

FIG. 47 is a schematic view of the operation of a "Fresnel mirror" usedin an analysis of the operation of an element of the electronic imageprojection system;

FIG. 48 is a schematic view of the one embodiment of a thin screensection of the electronic image projection system utilizing a Fresnelmirror and a rear screen;

FIG. 49 is a schematic view of a section of the electronic imageprojection system utilizing two Fresnel mirrors and a rear screen;

FIG. 50 is a schematic view of an alternate embodiment of a section ofthe electronic image projection system in which a section of anelliptical reflector is used to capture light that is not captured by aspherical reflector and a condenser lens to bring the light to a focusat the same point at which the aspheric condenser lens brings light tofocus for use in projection;

FIG. 51 is a schematic view of an element of the electronic imageprojection system referred to as a Fresnel parabolic reflector;

FIG. 52 is a schematic view of an embodiment of the electronic imageprojection system in which a full-color light valve is followed by alens array to create demagnified real images on the light valve pixelsin front of the lens array to allow for the projection of a full-colorimage in which the individual red, green and blue pixels are merged; and

FIG. 53 is a schematic view of four lenses in a lens array placed infront of a full-color light valve in an embodiment of the electronicimage projection system creating a real image of 24 pixel colorcomponents after the lens array.

FIG. 54 is a schematic depiction of a beam splitter cube and reflector;

FIG. 55 is a plot of the intensity of transmitted light through an LCDwith an input lens array as the LCD is rotated on a vertical axis;

FIG. 56 is a schematic depiction showing a light valve pixel hole withtwo input lens array elements and two output lens array elements;

FIG. 57 is a schematic depiction of how a large circular beam of lightcan be mapped onto a rectangular image forming element;

FIGS. 58A and 58B are top and side views depicting the use of efficientcomposite collector to gather majority of light from a uniform radiatingsource and prisms to fold sections of the collimated beam into the imageforming element;

FIGS. 59A and 59B schematically depict light emanating from a pointwhich is collimated by a Fresnel lens and folded by Fresnel prisms intoan image forming element;

FIGS. 60A and 60B depict plots of light intensity in the X and Ydirections of a sample system on the screen obtained with thearrangement of FIG. 59.

FIG. 61 schematically depicts a display system wherein three lightsources illuminate n image forming element after being collimated by alens;

FIG. 62 schematically depicts a layout to reduce the dimension of aprojector.

FIG. 63 schematically depicts a further layout which reduces thedimensions of a projector;

FIGS. 64A and 64B schematically depict embodiments of "Fresnel MacNeillepolarizers" of the invention;

FIG. 65 schematically depicts yet another layout which reduces thedimension of a projection;

FIG. 66 schematically depicts an embodiment of a projection system;

FIGS. 67A and 67B depict the path light takes through a projectiondisplay system embodiment using a light tunnel;

FIG. 68 depicts a non-imaging concentrator and spherical mirrors tocreate a collimated beam of light to illuminate an image formingelement;

FIG. 69 depicts a double input lens array system with an image formingelement;

FIG. 70 schematically depicts an alternate embodiment of the inventionusing two input lens arrays before the image forming element;

FIG. 71 schematically depicts a method of directing differently coloredlight beams to their respective pixels;

FIG. 72 schematically depicts an alternate method of directingdifferently colored light beams to their respective pixels;

FIG. 73 schematically depicts another method of separating light intocolors using dichroic mirrors;

FIG. 74 depicts another embodiment of the invention using two input lensarrays to direct colored light to respective pixels;

FIG. 75 is a schematic view of a technique of utilizing mirrors tocompensate for path length differences of beams travelling from thelight valve to the projection lens;

FIG. 76 depicts a preferred arrangement for making a color image-formingelement;

FIGS. 77A and 77B is a schematic view depicting light from a sourcerearranged to fill in weak or dead spots in a beam;

FIG. 78 depicts schematically a "Fresnel MacNeille polarizer";

FIG. 79 depicts another type of "Fresnel MacNeille Polarizer";

FIG. 80 depicts yet another type of "Fresnel MacNeille Polarizer";

FIG. 81 depicts still another type of "Fresnel MacNeille Polarizer";

FIG. 82 depicts a further another type of "Fresnel MacNeille Polarizer";

FIG. 83 depicts an embodiment showing how light from two sources can becombined to produce one linearly polarized beam;

FIG. 84 depicts an embodiment showing how light from two sources can becombined to produce one randomly polarized beam; and

FIG. 85 is depicts still a further embodiment of a "Fresnel MacNeillepolarizer".

DETAILED DESCRIPTION OF THE PREFERRED AND ALTERNATE EMBODIMENTS

The present invention is directed to A HIGH EFFICIENCY LIGHT VALVEPROJECTION SYSTEM. This overall system was devised to overcome theproblems of video display systems and to meet the objectives delineatedin the "Background of the Invention" section.

The most promising technology available to circumvent CRT problems islight valve technology. This technology uses an external light sourceand a "light valve," which modulates the light source, imposing image ordata information on the light beam, so that the beam can be projectedonto a viewing surface. Utilizing the same strategy as in a CRTprojection system, a light valve projection system can be constructed toproduce a brighter image than a CRT projection system. Such a systemcould also be produced to display black and white, monochromatic, orfull-color images.

Of all known light valve video display systems, the one which presentsthe greatest potential for solving the problems associated with CRTs isthe LCD with a conductive matrix for addressing, utilized intransmissive or reflective mode, taking advantage of thepolarization/rotation, birefringence, or scattering capabilities of theliquid crystals. Various changes must however be made to current videodisplay designs which use electronic multiplexing to eliminate thecurrent problems. Although LCD technology is preferred at this time,most of the present invention is applicable to light valve technology ingeneral and is to be interpreted with that broader view in mind.

FIG. 1 shows three light valves, one displaying red 110, one green 111and one blue 112 picture data, each light valve illuminated with lightof the appropriate color (100, 101, 102). The red light from source 100is collected by condenser 120, collimated by collimating optics 130 andprojected by projection optics 140 which focuses a red image on screen150. Similarly, the green and blue images are projected and made toconverge on the screen, forming a full-color image. The disadvantage ofthis full-color system, however, is that adjustments must be made to theoptics to converge the images whenever the projector or screen is moved.The need for convergence is eliminated in the present invention by theuse of dichroic mirrors and a single projection lens as schematicallyshown in FIG. 2. Red image information from light valve 200 reflects offfront-surface mirror 201 to dichroic mirror 204 which reflects red lightbut passes blue and green light. Blue image information from LCD 220reflects off front surface mirror 202 and then off dichroic mirror 203,which reflects blue light but permits green light to pass and thenpasses through dichroic mirror 204. A totally registered full-colorimage is thus projected by projection optics 205 onto screen 206.Convergence is always perfect, regardless of repositioning of theprojector or screen. The same invention can be applied to making a CRTprojector alleviate convergence problems.

If a picture is to be a mosaic of red, blue and green pixels, each pixelmust acquire a precise amount of current to reproduce the brightness ofeach picture element's originally broadcast brightness, as well as itscolor rendition. Although present LCD TV displays using electronicmultiplexing produce a satisfactory small image, when such images areprojected to a large picture, the transmitted light never reaches zero,causing low contrast. Additionally, with electronic multiplexing,cross-talk and electronic "bleed-through" to neighboring pixels reducesresolution and color fidelity. Furthermore, light is wasted and thepicture appears dim with each pixel being turned on for only part of ascanning field. The image cannot be refreshed sufficiently and soflicker, as well as brightness efficiency, is dependent on thepersistence of the LCD, which is not adjustable.

To solve the above problems, applicant's system can include a lightvalve in which the data used to address each pixel is stored, causingthat pixel of the light valve to remain activated the desired amount oftime until new data is received, dictating a different value for thatpixel. The data may be stored by various means, but preferably in acapacitor which is disconnected from the charging circuit immediatelyafter it is charged so as to remove the path for capacitive discharge.

Network analysis shows that when a given pixel is addressed through itsX and Y conductors, one-third of its addressing voltage will also appearacross other pixels. Since liquid crystal materials are fairly linear,this results in partial activation of incorrect pixels with false data.This can be alleviated by adding means to restrict the liquid crystalfrom being activated by increasing the threshold voltage of the liquidcrystal, making its response to voltage non-linear, or by adding aswitching mechanism to disconnect the pixel from the circuit until it isto be addressed. The preferred way to accomplish this is by adding a"switch" to each pixel, creating what is known as an "active matrix"addressing system.

For instance, as shown in FIG. 18, an X-Y matrix of pixels made oftransparent conductive material, such as indium tin oxide, is coated onthe inner faces of a glass container which is filled with liquid crystalmaterial 1800. Each pixel in a given horizontal row on one face is putin contact with a gas such as helium in a reservoir 1810 which requiresa threshold voltage to ionize it and create a path for current flow tothe pixel electrodes in the row. The corresponding pixel electrodes 1820on the opposite glass plate are connected, for instance, to video signalinputs along vertical lines. When a threshold voltage is reached atwhich the gas for a given row on the first glass plate becomes ionized,the video signals applied in vertical columns to the corresponding pixelelectrodes on the opposite glass plate charge those pixel electrodes,the liquid crystal material between the plates acting as a dielectric toform a capacitor. Immediately thereafter, removal of the thresholdvoltage necessary to ionize the gas leaves the pixel electrodecapacitors along the horizontal row charged the required amount tomaintain the polarization rotation through the liquid crystal materialalong that row until new data is available to replace the data alreadystored.

Alternately, an "active matrix" can be created by the deposition of athin film transistor next to each pixel and by using a storage elementat each pixel. Each transistor receives a gate signal, turning it on andallowing the conduction of a video signal voltage to the pixelassociated with the transistor that is turned on. When the transistor isswitched off (by removing the gate signal), the pixel electrodes withliquid crystal material between them act as a capacitor storing thecharge and maintaining the state of activation of the liquid crystalmaterial until changed by a new signal. An additional capacitor can beadded to maintain the charge if the liquid crystal material has too muchcharge leakage.

This way, each pixel can be addressed, turned on (to transmit or reflectlight) and will remain on until data for the next frame is presented.With this system, flicker can be eliminated as in a progressivelyscanned picture. Each pixel will be on for the entire length of a frame,immediately changing to the appropriate level of transmissivity orreflectivity for the pixel in the next frame. Each pixel will be on (thedesired amount) all the time, allowing the highest throughput of lightfrom the external light source. State of the art methods of depositionof semiconductor material can be utilized to mass-produce such an activematrix system. Similarly, in addition to active matrix addressing oflight valves such as LCDS, other methods, including scanned electron andscanned laser beam addressing can be utilized in a light valve within aprojector.

The light valve can be used in conjunction with direct projectionoptics. A general overview of the present invention is depictedschematically in FIG. 17 as comprising a light source 1700 from whichemerges a beam of light, collimating optics 1710 which collimates thebeam, including a spherical or parabolic reflector 1720 which reflectsthe beam, a condensing lens 1730 which focuses the beam forward andcollimating lenses 1740 which again collimate the beam. The light valve(or light valves) 1750 is illuminated by the collimated beam, creating afull-color optical image thereupon. Projection optics 1780 then focusesthis image onto a viewing surface 1790. To improve the quality of theprojected image as explained further herein, subsystem 1760 is used tosuperpose pixels of color triads forming full-color pixels with spacesbetween them and subsystem 1770, also explained herein, may be used tofill in the spaces between pixels.

An active matrix light valve made by the deposition of thin filmtransistors also has significant drawbacks. The chances for defects suchas shorts and opens abound because of the small feature dimensions, themany layers of deposition and the high density of conductive paths,transistors, and other features in such light valves. A simple defectcan cause an entire row of pixels to be permanently on or permanentlyoff and can render an entire display useless since defects projectedonto a screen become very noticeable and unacceptable. The display yieldaccordingly goes down dramatically as the resolution and/or size of thedisplay increases and the cost of an acceptable display dramaticallyincreases. Techniques such as redundant transistors at each pixel,redundant conductive pathways and the use of a laser to eliminateshorted transistors or pathways have been devised to compensate for suchdefects. However, even with these techniques, many defects are notcorrectable, keeping yield low and costs high.

Applicant's technique of placing two otherwise unacceptable displaysback-to-back with appropriate display drivers greatly increases theyield and reduces the cost of producing active matrix displays. (SeeFIG. 19.) Although each display 1910 and 1920 separately is unacceptablebecause of its relatively few uncorrectable defects, 1911 and 1921,respectively, two rejected displays can be combined where the defects inone do not correspond to the defects of the other. The output faces orthe input faces of the two displays must be facing one another in aconventional LCD which has a twist angle of 90 degrees (unless a halfwave plate is placed between them). This way, vertically polarizedlight, for instance, entering the input face of the first display isrotated 90 degrees by the liquid crystal material when no current isapplied, exiting as horizontally polarized light. It can now enter theoutput face of the second display and be rotated by the liquid crystalmaterial to become vertically polarized and exit the input face of thesecond display. Consequently, no polarizer need be placed between thedisplays.

Although transmission light valves are preferred in applicant's system,reflection light valves could be used as well. When utilizing liquidcrystals as the active medium, use of the twisted nematic effect iscurrently the most common method of modulating the light to produce asatisfactory image. However, use of the twisted nematic effect does notwork well in a reflection light valve. This is because polarized lightwhich enters the light valve (polarized, for example, in the verticaldirection) will rotate 90 degrees, hit the rear reflector and rotateback 90 degrees upon passing a second time through the twisted nematiccell. Thus the light will exit predominantly as it went in with theinitial polarization. When there is a signal causing a voltage to beimposed on the liquid crystal material, the nematic liquid crystals willbecome perpendicular to the cell faces to some degree (depending onvoltage), losing their twisted orientation with respect to the light.Thus, light entering the cell will pass through the cell and reflectback out unaltered. Thus, whether or not a voltage is applied, lightcomes out of a reflective cell unaffected by the twisted nematic effect.

A reflective liquid crystal cell can work utilizing scattering or thebirefringence of the liquid crystals. A reflective active matrix lightvalve can be constructed in many ways. For instance, a single siliconchip can be made into an active matrix utilizing state of the artsilicon chip fabrication technology such as proposed by Hughes in the1970s with reflective pixel electrodes on the silicon chip made of amaterial such as aluminum. The opposite faces of the cell can be made ofglass with transparent indium tin oxide pixel electrodes.

Utilizing the scattering effect (see FIG. 20), light which enters thecell 2000 can hit a specularly reflecting back surface and reflect outof the cell for focusing, for instance, through an aperture 2010, as ina Schlieren type optical system. When a voltage is applied in a givenarea, light is scattered in proportion to the voltage, preventing itfrom being focused through the aperture on to the screen 2020. To makeuse of the birefringence of liquid crystal molecules, a cell can beconstructed wherein the liquid crystal dipoles are oriented eitherparallel or perpendicular to the faces of the cell or somewhere inbetween, depending upon the applied voltage. In this case, polarizedmolet entering the cell when the molecules are oriented perpendicular tothe faces of the cell, will emerge from the cell after reflection from arear reflective surface with its polarization unchanged. However, withthe proper cell thickness when the dipole molecules are completely orpartially parallel to the cell's faces, the birefringence of the liquidcrystal molecules will cause the liquid crystal material to act like aquarter wave plate of varying efficiency. Thus, after passage in and outof such a reflective cell, polarized light will have its plane ofpolarization rotated, to some degree (up to 90 degrees) depending uponthe voltage applied (double passage through the cell making the celloperate as a half wave plate).

Heat and IR radiation generated by the required projection bulb aresources of lowered resolution and contrast, as well as color andgray-level distortion, and could damage the light valve. Heat and IR,like the light, irradiates the light valve in a Gaussian-like pattern,causing a "hot spot" in the center of the light valve. Even if thedamage threshold is not reached, image degradation could still occurbecause the light valve expands, increasing the distance light musttravel through it. When the polarization rotation effect is used, therotation of the plane of polarization of the light passing through thelight valve could change, throwing off contrast, resolution and colorand gray-level rendition in a Gaussian-like pattern.

Several steps may be taken to deal with the detrimental effects ofheating of the light valve. First, all optics including the light valve,should be mounted with good contact to large heat sinks, as is done, forinstance, with power transistors. Optics in the system, including thelight valve windows, can be made of or coated with substances such asdiamond and sapphire, which have excellent optical qualities andunusually high heat conductive capabilities. Additionally, all opticscan be coated with material of proper thickness, such as is done fordichroic reflectors to reflect the infrared (IR) spectrum. IR reflectingmirrors and heat absorbing glass can also be used in the optical path.Additionally, a fluid means such as a liquid or gas in a container,consisting of a large body of index-matched high-boiling-point fluid(liquid or gas), can be used for further cooling. This fluid may bestatic or circulating within a contained area and placed in contact withthe components to be cooled. Alternatively, instead of transmissiveoptics, reflective optics such as optics made of metal can be utilizedfor further heat sinking and to suppress reflection at IR wavelengths(with anti-reflection coating for the IR).

Anti-reflection (AR) coatings can, of course, be used on all opticalsurfaces to reduce light losses due to reflection at those surfaces.Such surfaces include surfaces of lenses, hot mirrors, heat absorbers,polarizers, prisms and light valves such as LCDs, including the internalsurfaces of the glass faces of the light valves to reduce reflections atglass--ITO boundaries glass--liquid crystal boundaries, ITO liquidcrystal boundaries, etc.

Cooling fans may be used to cool the light valve as well as the othercomponents of the system. Ducts and narrow tubes can be used to providecooling to specific spots. However, a fan can pose a noise problem,particularly noticeable when the audio volume of the system is at a lowlevel, particularly in a small room. To suppress the noise, an "airbaffle" may be used between the fan and the outlet of, for example, ahousing for various components of the invention. FIG. 16 shows a soundsuppression system, comprising fan 1600 resting on platform 1620.Airflow blockers 1630 force the air to traverse a curved path withdeflection prior to exiting the housing through outlet 1640. Thesurfaces from which the air and sound reflects are covered with soundabsorbing materials, greatly reducing the noise entering the listeningenvironment. Since some noise will still be present at outlet 1640, afurther measure may be taken for noise reduction. This measure couldcomprise microphone 1650 which picks up the remaining noise and sends itto an amplifier which inverts the phase of the noise by 180 degrees. Theinverted noise is played back through speaker 1660. By properlyadjusting the volume and phasing of the amplifier, the remainingperceived fan noise could be substantially reduced and made practicallyinaudible.

Depending upon the brightness of the light source utilized and thephysical and economic constraints of a given system, some significantGaussian-like heat pattern could remain at the light valve and couldchange with time as overall heat builds up during operation. Anelectronic approach can therefore be used in conjunction with the otherrecited remedies to eliminate the problem. Modifying the electronicfield in opposition to temperature effects will substantially cancel thedistortion resultant from such effects, since the degree of rotation ofthe plane of polarization of the light is not only dependent on thethickness of the light valve that it passes through, but also upon theamount of applied electric field. The result will be uniform performanceacross the light valve. Such a system would use a bias voltage applieddifferently to different pixels, distributed in a Gaussian-like patternacross the light valve. A thermistor or other temperature-sensingdevice, placed at the light valve, can monitor overall average lightvalve temperature, adjusting the Gaussian-like bias voltage distributionas the temperature fluctuates, using an electronic feedback circuit. Foreven more accurate temperature control, a thermistor-type device can bedeposited next to each pixel in the space between the pixels toindependently control the heat-compensating bias of each pixel.

An "active matrix" will allow for more brightness in the projected imagethan a multiplexed array and less heat will be generated for a givenlevel of brightness. Addressing each pixel separately in this wayeliminates cross-talk. However, all the conductive pathways,transistors, and capacitors create substantial "dead space" betweenpixels. These dead spaces are generally in the area of "overlap" whereelectric fields from neighboring pixels could co-mingle and producefalse data, reducing contrast and distorting the color mix. Placing anopaque, black, reflective or other covering over these areas serves atleast three purposes: it stops passage of improperly modulated andunmodulated light from passage to the screen, protects thesemiconductors from damage due to irradiation from the intense light andheat and reduces the chance of discharge of pixels. The covered area maybe a fraction of the size of a pixel.

As an alternative to using three light valves in a projection system toproduce full-color, there are several ways to construct a full-colorprojection system using a single light valve. A simple, compact andinexpensive full-color video projection system may be constructed usinga single "full-color" light valve. Previously full-color, direct-viewvideo image displays not using projection had been constructed with asingle "full-color" LCD. When such images were enlarged by projection,however, several problems explained herein become apparent.

In a standard CRT-based TV system, red, blue and green pixel data aresent to adjacent red, blue and green phosphor spots on the CRT face.Analogously, in a direct-view full-color LCD TV system, red, blue andgreen pixel data are sent to adjacent areas of the LCD. These areas arethen covered by red, blue and green filters to appropriately color thelight passing through those LCD pixel elements. FIG. 15a depicts asimple arrangement of color pixels in which pixels of a given color arelocated above one another creating vertical color stripes. Threehorizontally adjacent pixel areas make up a pixel triad which representsa single, full-color pixel from the actual image. FIG. 15b depicts analternate arrangement of pixels in which the three pixels of a colortriad are arranged to form a triangle. In the preferred single lightvalve embodiment, such a full-color light valve can be placed atposition 1750 in FIG. 17 to produce a full-color image.

In one embodiment, a single light valve 2100 may be divided into threesections. The red image for instance, can be made to electronicallyaddress the left 1/3 of the light valve panel 2110, while the electronicdata corresponding to the green component of the image addresses thecenter 1/3 of the light valve 2120, and the electronic data representingthe blue component of the image can address the right 1/3 of the lightvalve 2130. (See FIG. 21.) Light from these three images can then beoverlapped and projected through projection optics to the screen. Sincethe projection lens 2220 has a given focal length, it must be placedapproximately its focal length away from each. color component image.(It must be optically equidistant from each image.) This can beaccomplished in a number of ways. One or more lenses can be positionedjust after the light valve 2100 to adjust the focus of one or more ofthe three images through the same projection lens even though the threeimages may traverse different light paths. (See FIG. 22.) For instance,correction lens 2201 can correct for the distance difference in thestraight-through path as compared to the reflected paths. Alternatively,path lengths can be matched by the appropriate use of mirrors, as forexample, depicted in FIG. 23 or, preferably, FIG. 75. In FIG. 75, 7510,7520, 7530, and 7540 represent first-surface mirrors or prisms toreflect the beams while 7550 and 7560 are dichroic mirrors. Light fromIFE 7570 is expanded by lenses 7580 to create an image with the desiredaspect ratio. Proper selection of angles (such as A1=67.38 degrees andA2=36.87 degrees) will allow all beams to traverse equal paths forcombination into a single full-color beam. Obviously, the IFE could bedivided into sections in different ways, such as into horizontalsections or divided in half with one section divided in half again, etc.As mentioned earlier, reflection optics, including a reflection lightvalve, can be used to produce the full-color video image. An example ofthis type of set-up with a single light valve is shown in FIG. 24.

In this set-up, light from light source 2400 is collected and collimatedby condenser optics 2410. After passage through a quarter wave plate2420, the light enters a MacNeille beam splitter cube 2000. S-polarizedlight reflects from the internal face within the cube to front-surfacemirror 2430. This reflects the S-polarized light back through the cube,through the quarter wave plate, back through the condenser optics andlight bulb, and back through the quarter wave plate. At this point, theS-polarized light, having passed twice through the quarter wave plate isrotated 90 degrees to become P-polarized light. It can now pass throughthe cube, resulting in utilization of a majority of the source light,even though plane polarization is performed.

Dichroic mirror set-up 2440 separates the light into red, green and bluebeams which reflect from path equalization mirrors 2450 and illuminatethree sections of light valve 2100, which is addressed with threecolor-component images. The light reflects from the light valve andretraces its path to the MacNeille prism. Light which should appear inthe projected image is converted by the light valve from P-polarizedlight to S-polarized light. It therefore reflects from the inner surfaceof the cube and exits through the projection lens 2220 to the screen.Non-image light remains P-polarized and passes through the cube and isreinjected into the system, making the projected image somewhatbrighter. A birefringence transmission light valve with a mirror behindit could also be used in this arrangement.

In conventionally made LCDs, color filters are deposited within thecavity of the LCD. This must be done because any difference in physicallocation of the actual LCD pixels and the color filters coloring themwill produce a parallax difference which will be perceived asmisregistered or incorrect colors when viewing a direct-view LCD fromany angle aside from head-on.

Since the space between the glass plates forming an LCD is typicallyless than 10 microns, the deposition of color filters requires a highdegree of thickness control as well as color transmissivity and overalltransmissivity uniformity in such thin coating thicknesses. In addition,high efficiency filtering must be used to eliminate the possibility ofcontaminating particulate matter in the coating chemicals which may beon the order of or larger than the space in between the glass plates.

Projection, however, presents the unique situation in which a lightvalve can be illuminated with substantially collimated light and viewedon a screen from all angles even though light passes through the lightvalve substantially in a parallel direction eliminating any possibleparallax error. This means that the making of full-color light valvesspecifically for their use in projection will allow the use of externalcolor filters whose thicknesses do not have to be as preciselycontrolled. Also, being placed outside of the light valve cavity reducesthe risk of contamination as well as the complexity and thus the cost ofproduction of light valves for that purpose.

Using a "full-color" light valve can create another problem which,although not very noticeable on small displays, creates major problemsin a large image. This problem results in a poor contrast ratio and poorcolor fidelity. To understand and correct this problem the workings of afull-color LCD display must be analyzed.

The following discussion explains the nature of the problem. Thetransmitted light intensity (TI) from a twisted nematic liquid crystaldevice, under no applied voltage, with a crystal thickness (d) for anygiven wavelength (λ), is dependent on the refraction anisotropy (Δn) andthe liquid crystal twist angle (θ). TI can equal zero for only a fewunique simultaneous combinations of values for these parameters. Thismeans that except for very specific combinations of wavelength (λ) andthickness (d) for any given crystal, zero transmitted intensity or true"black" will not occur. Thus, if the anisotropy, twist angle, andcrystal thickness are fixed, as they are in a conventional light valvesuch as an LCD (consisting of liquid crystal between two flat plates),only one color can go to black at a time. If a voltage is applied,changing the light rotation, then a different color can go to black.This non-linearity eliminates the possibility of true black in allcolors simultaneously (and thus limits possible contrast) and sinceperceived color is produced by addition, this eliminates true colorfidelity.

To further illustrate this problem, the dashed curve of FIG. 10 showsthe transmitted intensity over the visible spectrum of a standardfull-color LCD with a given thickness. FIG. 11, plot A shows thenon-linear transmittance variations for the three wavelengths used in afull-color LCD of uniform thickness plotted against the voltage. Whenred transmission, for instance, is at a minimum, blue transmission isover 10 percent and green transmission is about 5 percent. Having notrue black results in a low contrast ratio which is one of the mainproblems with today's LCDs. To solve this problem, one of the variablesgiven above must be modified to produce the desired transmissivity for agiven signal voltage. This can be done by electronically biasing thepixels, which are addressed with data corresponding to two of the colorcomponents (such as red and green). This would cause the nettransmissivity through the red and green pixels to equal thetransmissivity of the blue pixels, when no signal voltages are presentfor any pixels. With proper selection of d, all colors will be at aminimum.

Alternatively, the crystal thickness (the space between the platesencasing the liquid crystal) can be selected under each color filtersuch that at exactly zero (signal) volts, the proper rotation is imposedon the polarized light for the specific wavelength transmitted by thatcolor filter. By doing this for each of the three sets of color filters,the minimum amount of light for each color will be transmitted with novoltage applied. This, again, will provide a blacker black and thus ahigh contrast. This result is accomplished, for instance, if steppeddeposition or etching of one plate is done to produce steps asillustrated in FIG. 12.

By using a light valve with such a "stepped thickness" cavity, thecrystal thickness-wavelength combination will allow true black for allthree colors simultaneously and a linear relationship between appliedvoltage and transmitted intensity for all colors simultaneously. This isdemonstrated by FIG. 10 (solid line) where transmission is nearly zerofor all colors simultaneously with no voltage applied and in FIG. 11,plot B, where the transmission for all colors varies with voltagesimultaneously.

In applicant's demonstration model, using a "stepped thickness" cavityresults in a contrast ratio as high as 100:1 and color fidelityapproaching that of a CRT. This high color fidelity can be seen in theCIE diagram of FIG. 13 in which the dashed line represents thechromaticity of conventional multi-color LC displays, the dotted linerepresents the chromaticity of an LC color display with varying crystalthicknesses and the solid line represents the chromaticity of aconventional CRT.

The small, closely packed red, blue and green spots of light that makeup a direct-view image create the illusion of color in a scene as theyare supposed to appear. However, when this image is magnified byprojection, each adjacent red, blue, and green pixel no longer merges toproduce properly colored areas. Instead, they appear as disjointed red,blue, and green areas, detracting from the appearance of a naturallycolored image. Furthermore, dead spaces between adjacent pixel areas inthe light valve are magnified as well, further creating a disjointed,disruptive, unnatural looking image. The appearance of disjointed red,blue and green spots instead of actual colors in a full-color lightvalve can be eliminated by various methods.

The concept of depixelization, or substantially reducing the perceivedappearance of dots, lines, pixels, dead spaces, or other suchnon-information bearing portions of the image, as proposed herein, alongwith suggested methods to accomplish it, is applicable to any displayedimage that contains such areas, whether it is a projected or adirect-view image display. Although the "smoothing effect" is mostnoticeable in a large display with large pixels, it still improvesperceived image quality even when the displayed image is small and/orcontains small pixels.

The preferred method of eliminating them in the projected image,utilizing a single, full-color light valve, entails the use of lensarrays. FIG. 52 shows a full-color light valve 5200 with red, green andblue pixels arranged in horizontal rows 5210. The rows are preferablyarranged so that each succeeding row is offset by 11/2 pixels from theprevious row, although many other arrangements are possible. A lensarray 5230 is placed in front of the light valve and behind theprojection lens 5240. The lens array could comprise spherical lenses,although cylindrical or other types of lenses could be used, each ofwhich is 1/2 the width of a pixel on the light valve. The curvature ofeach lenslet and the distance between the lens array and the light valvecan be chosen so that each lenslet 5250 creates a demagnified real imageof a portion of the light valve, floating in space, slightly in front ofthe lens array, between the lens array and the projection lens. Otherarrangements are, of course, possible.

As shown in FIG. 52 (inset) 5250, the real image produced by a singlelenslet contains data from 6 pixels. These 6 pixel images come from twohorizontal rows with 3 pixels on top and 3 pixels below. Other lenssizes and curvatures could be used and each real image could contain adifferent number of pixel images while still producing essentially thesame result. The addition of the lens array separates the planes of bestfocus of the red, green, and blue pixel data and the image informationdisplayed on the light valve. The projection lens focuses through thelens array onto the plane of the best image focus, near the plane of thelight valve. Since 4 lenslets 5300 (see FIG. 53) occupy the same amountof space as a single pixel 5310 and each lenslet produces an image of 6pixels in this case, the image focused on the screen of a single pixelwill be the superposition of 24 red, green and blue dots. These dots,however, are not 24 different pixels, but contain the data from only 6pixels on the light valve (which may correspond to only two pixels inthe actual scene). The 24 dots that superimpose to create the image ofthe next pixel contain some of the same information as the previous 24dots or some portion of the same dots and some new ones. Consequently,each adjacent pixel image is a weighted average of approximately 2triads, causing only a slight reduction in resolution. However, sinceeach newly created pixel image is an out-of-focus superposition of 24dots, its colors combine to produce a net uniform color. Thus, afull-color image is still displayed with correct colors in the correctlocations to a sufficient degree of accuracy so that the image appearsessentially unchanged from that projected without the lens array, exceptthat individual red, green and blue dots are no longer visible. Thisblending process also eliminates the appearance of any spaces betweenpixels. This combined function eliminates the appearance of pixelsaltogether. Use of an anamorphic lenslet profile, or the opticalequivalent formed by crossed lenticular lenses is preferred so that the"blur" is only a mix of one red, one green and one blue pixel.

When constructing a rear-screen display unit, an additional flexibilityis provided since the screen is built into the unit. This allows for theaddition of optics just before the screen. If the image projected onto arear screen has individual red, green and blue pixels, a lens array asdescribed, which has for instance twice as many lenses as there arepixels in each orthogonal direction, can be placed near the focusedimage that is to hit the screen. As explained above, each lens elementcan create a demagnified image of one or more triads in space. A secondlens array with the same number of lenslets as there are pixels can thenfocus a blended image of the new pixel onto a nearby screen surface(being focused on a plane near the original image plane, not on theplane of real images of the pixels). As before, the individual colorpixels will be blended into full-color pixels.

Alternatively, a single lens array can be used if it is made in aspecial way. The single array should have the same number of lenslets asthere are individual colored pixels. The array is placed after the imagethat is to be focused on the screen. Two of every three lenses in thearray also have a built-in wedge so that the images of a triad will allbe focused onto a nearby screen overlapped, creating full-color pixels.The wedges can, of course, be separate from the lenslets. These last twotechniques can also be applied to a CRT or any imaging device whichnormally displays individual red, green and blue pixels.

Another method of creating full-color pixels entails the use of narrowangled prisms or wedges. As shown in FIG. 28, these two wedges can beplaced with a clear space between them at any place in the system aslong as they are not placed too close to the light valve. Since thelight distribution is usually Gaussian, more light is concentrated inthe center. To make all three images equal in brightness, the clearcenter section should therefore be smaller than each wedge section.Alternatively, to produce a more uniform image, the wedges can bedivided into thin sections and interdispersed with clear spaces. If thewedges are placed somewhere between the light source and the lightvalve, they will create the equivalent of three very close lightsources, illuminating the light valve from slightly different angles.This will create three slightly displaced images on the screen.

The wedges can also be positioned somewhere after the light valve, suchas after the projection lens. Such positioning will create three imageson the screen, each slightly offset from the other.

If the wedge angles are properly chosen based on simple geometricalconsiderations, the images will be offset by the width of one pixel. Thered pixels of one image will then be superimposed on the neighboringgreen pixels of the second image, which will be superimposed on theneighboring blue pixels of the third image, creating full-color pixelsin which individual red, green and blue pixels will not be visible. Thistechnique will work well in most areas since most groups of three pixelsin an image will most likely have the same color value. The only placethis technique will create a slight problem is at the boundary betweentwo very different areas. At the boundary, when there is an abruptchange in color and/or brightness, two of the pixels that are overlappedon neighboring pixels will be overlapped on neighbors that should havedifferent values and therefore a noticeable distortion will becomeapparent, creating a more jagged looking edge at the boundaries of theviewed image. The larger the areas of constant color within a scene, theless noticeable this will be.

Another method to eliminate the appearance of the individual coloredpixels is by the use of a dichroic mirror system as depicted in FIG. 9a.Assuming the pixel arrangement of FIG. 15a, individual red, blue andgreen pixels can be made to overlap by the following arrangement:collimated light 901 passes through the full-color light valve 902 andhits dichroic mirror 903 which reflects only the blue image. Theremaining red and green images pass through dichroic mirror 903, hittingdichroic mirror surface 904 which reflects only the red image, allowingthe green image to pass through. The blue image reflects off frontsurface mirrors 910 and 911 and then off dichroic mirror surface 905which reflects only blue light. Here the blue image rejoins the greenimage. By adjusting front surface mirrors 910 and 911 the blue pixelscan be made to overlap the green pixels. The red image reflects offfront surface mirrors 920 and 921 and then off dichroic mirror 906 whichonly reflects red light. At 920 and 921, the red pixels can be made tooverlap the already joined blue and green pixels. The path lengths couldbe matched using a compensating lens as described herein or additionalmirrors as also described herein. At this juncture, we have a full-colorimage with large spaces between pixels as illustrated in FIG. 4.

If individual colored pixels are arranged on the light valve as shown byFIG. 15b, in which a color triad forms a triangle, bringing the red andblue pixels together, as described, will not allow them to superimposeon top of the proper green pixels since the proper green pixels arevertically displaced from their corresponding red and blue pixels.Consequently, this type of pixel arrangement could use an additionaldichroic mirror path similar to the paths used by the red and bluelight. This is depicted more clearly in FIG. 9b, which is a side view ofthe system in FIG. 9a modified to include an additional light path.Collimated light 901 passes through full-color light valve 902 asbefore. However, the distance between light valve 902 and dichroicmirror 903 is increased to allow for the insertion of dichroic mirror950 which reflects green light and transmits red and blue light. Asbefore, 903 reflects blue light and transmits red light. Mirror surfaces904 and 905 are front surface mirrors. Mirror 906 reflects red light andtransmits blue light. As before, mirrors 910, 911, 920 and 921 are frontsurface mirrors. In addition, mirrors 960 and 970 are also front surfacemirrors. Mirror 980 is a dichroic mirror which reflects green light andtransmits red and blue light. By this modified arrangement, properseparation of mirror 910 from mirror 911 and separation of mirror 920from mirror 921 will still cause the overlap of the red and blue pixels.Additionally, proper separation of mirrors 960 and 970 will cause theproper green pixels to overlap the already joined red-blue pixel pairs.This overhead mirror arrangement may also be used with the color lightvalve whose pixel arrangement is depicted in FIG. 15a with the spacingbetween mirrors 960 and 970 adjusted to prevent vertical displacement ofthe green pixels since they are already in line with the red and bluepixels. The separate mirror path for the green light makes the distancetraversed by each color equal, which is important because the light,although collimated, still expands with distance traveled and theprojection lens must focus all three images simultaneously. Now theimage can pass through subsystem 930 which can be used to fill thespaces between pixels (as described elsewhere herein) for finalprojection by projection optics 940.

Alternatively, in FIG. 9a, mirrors 910, 911 and 920, 921 could be tiltedup or down to cause the red and blue pixels to superimpose on the propergreen pixel.

In another embodiment for the elimination of the appearance of red,green and blue pixels, depicted in FIG. 29, four special mirrors areused. Each mirror has clear spaces and mirrored areas. Two of themirrors 2910 and 2920 have ordinary mirrored areas coated, for instance,with silver or aluminum, which totally reflect light of any color. Oneof the special mirror's 2930 reflective coatings is dichroic andreflects blue light and transmits red and green light. The other specialmirror's 2940 reflective dichroic coating reflects red light. As seen inFIG. 29, the mirrored areas of the four-mirrors are positioned out ofphase with each other. On each mirror, the clear space between everytwo-mirrored spaces is equal to twice the width of the mirrored space.

Light from red pixel #1 2950 passes through the clear area of the firstmirror and reflects off the mirrored area of the second mirror downwardtowards the red reflective area of the first mirror. The red light isthen reflected upward, passing through the clear area of the secondmirror and then passes through the clear areas of the third and fourthmirrors.

The green light coming from green pixel #2 2960 passes through thedichroic mirrored area of mirror #1, passes through the clear area ofmirror #2, passes through the dichroic mirrored area of mirror #3 andpasses through the clear area of mirror #4 and is thus superimposed onthe light that came from the red pixel.

Light from the blue pixel #3 2970 passes through the clear spaces inmirrors #1, #2 and #3 and reflects off the mirrored area in mirror #4down to the dichroic mirrored area of mirror #3. This dichroic mirroredarea reflects the blue light upwards, superimposing it on the light fromthe red and green pixels. Thus, we have created full-color pixels withspaces between them.

In an alternate embodiment (see FIG. 30), two special mirrors are used.Each mirror has properly mounted 45 degree dichroic mirror sections. Thefirst mirror 3010 reflects red light and transmits blue and green, whilethe second mirror 3020 reflects blue light and transmits red and green.In the arrangement, red light from red pixel #1 reflects off two reddichroic surfaces upwardly through the second blue dichroic mirror 3020.Green light from green pixel #2 goes straight upwards, passing throughboth the red and blue dichroic mirrors. Blue light from blue pixel #3passes through the clear space in the first mirror and reflects off twoblue dichroic mirror surfaces in the second mirror, sending it in anupward direction. As before, this arrangement superimposes the lightfrom the red, green and blue pixels into a single beam, creatingfull-color pixels separated by spaces.

Three special "mirrors" (see FIG. 31) are used in another method ofcreating full-color pixels. Each "mirror" consists of properly placed 45degree dichroic mirror sections. The first mirror 3110 is a red dichroicmirror, reflecting red light but transmitting green and blue light. Thesecond mirror 3120 is a green dichroic mirror, reflecting green lightbut transmitting red and blue light, and the third dichroic mirror 3130is a blue dichroic mirror reflecting blue light but transmitting red andgreen light. In this arrangement, red light from red pixel #1 reflectsoff the two red dichroic mirrors 3110 into the upward direction passingthrough the green and blue dichroic mirrors. Green light from greenpixel #2 similarly makes two reflections from green dichroic mirrors3120 reflecting it in an upwards direction and superimposing on thelight from the red pixel. Light from the blue pixel #3 also reflects offtwo blue dichroic mirrors 3130, upwardly superimposing it on the lightfrom the red and green pixels. Again, full-color pixels are createdseparated by spaces.

Various other arrangements can be devised, also utilizing dichroicmirrors, to superimpose red, green and blue pixels. As another example,the image, emerging from the projection lens can reflect from two"sandwich" surfaces are separated by a preciser spacing. As an example,the first mirror sandwich can superimpose the red pixels onto the greenpixels by the action of a red dichroic mirror (see FIG. 60). The secondmirror sandwich can then superimpose the blue pixels on the resultingred and green pixels to form full-color pixels. Large spaces (2 pixelswide) will be formed between resulting full-color pixels which can beeliminated as explained elsewhere herein.

Visibility of red, green and blue pixels could also be eliminated byusing a single, relatively low resolution light valve with a "time-sharescanning" technique. By dividing time into small segments, each withdifferent data presented to the screen, the eye will integrate the dataover time, seeing the sum of the data, as if each different datapresentation were being projected simultaneously onto the screen.However, time-sharing of visually-presented data must be done properlyor else artifacts, such as flicker and reduced image brightness, willbecome apparent to the viewer.

As an example, if the light valve is addressed with red informationonly, and only red light is projected through the light valve duringthat time, followed by the green and blue images similarly projected,the viewer will perceive a full-color image. However, since a standardvideo image provides 30 frames per second and since flicker is almostvisible to many viewers at this frequency, dividing time into segmentsas described, would produce 10 images per second for each color,creating a noticeable color flicker. In addition, if a large area isonly one color (as often happens in real life), then the entire areawill be black for two out of every three time segments, decreasingperceived brightness to one-third and creating a strongly pronouncedflicker of the entire area. This problem was studied in great detail inthe early days of color television, when CBS attempted to develop theirsequential color system, using a spinning color wheel in front of amonochrome CRT. Another problem encountered when using this method is amarked decrease in image brightness, due to another factor. Since,during any given frame, only one color of light is projected on thescreen, two-thirds of the light emitted by the source is thereforeeliminated from every frame, and thus from the viewed image.

To eliminate these problems, a system can be set-up in the followingway. Firstly, the light valve is addressed as a full-color light valve,with pixels arranged in an alternating fashion in which every even rowcontains the pixels in the order of one red, one green and one blue,repeating throughout the line. Every odd line may contain pixels in thesame arrangement, but may be displaced some amount such as one andone-half pixels, with respect to every even line. This creates a morerandom appearing pixel pattern. For a single segment of time (such as1/30 of a second) the light valve is addressed in this fashion, andlight of the proper colors is sent to each pixel through a mosaic ofcolor filters (as previously described) or by the creation of a matchingmosaic of colored light beams, created for instance by multiple dichroicmirrors as described elsewhere, herein. For the next segment of time,the light valve is addressed with all color data addressing shifted byone pixel in a given direction. Simultaneously, the distribution ofcolored light beams addressing the light valve is shifted to correspondto the new positions of the colored data on the light valve by eithermoving the color filters or by appropriately vibrating mirrors in thedichroic-colored-reproduction system.

In this embodiment of time-share scanning, pixel #1 of the light valveis addressed with red data corresponding to pixel #1 of the image, forthe first segment of time. This produces a red data image in pixel #1 onthe screen during that segment of time. In the next segment of time, thecolor data locations, as well as the arrangement of the colored beams,are shifted so that pixel #1 on the light valve is now displaying thegreen data from pixel #1 in the original image. This green data frompixel #1 in the original image is now projected onto the same locationon the screen that displayed the red data for pixel #1 in the previoustime segment. Similarly, the blue data is projected to pixel #1 on thescreen in the next time segment, creating the illusion of a full-colorimage at every pixel location within 1/10 of a second. Any large area,which is one color only, now has one-third of its pixels on with thatsingle color during every time period (such as 1/30 of a second). Thus,the area appears that color all the time instead of being blacktwo-thirds of the time, as explained above.

With this arrangement, at least one of every three pixels sends light tothe screen all the time, assuming there is any light in that area in theimage. Utilizing the dichroic mirror method (described elsewhere herein)of dividing the light into multiple-colored beams in the properarrangements eliminates the problem of wasting two-thirds of the bulb'slight during any given time segment since all of the light is used inevery time segment.

As a preferred embodiment of "time share scanning," the light valve canbe addressed so that pixel #1 is always addressed with red data, pixel#2 is always addressed with green data, pixel #3 is always addressedwith blue data and so on. The illumination is fixed so that pixel #1 isalways illuminated by a red beam, pixel #2 is always illuminated by agreen beam, pixel #3 is always illuminated by a blue beam, and so on.However, in this embodiment, pixel #1 of the light valve is addressedwith red data from pixel #1 of the image in the first time segment andis then addressed with red data from pixel #2 of the image in the secondtime segment and is then addressed with red data from pixel #3 of theimage in the third time segment and then back to red data from pixel #1of the image, and so on, for all other pixels. The light exiting fromthe light valve before going to the screen reflects off a mirror. Thismirror is oscillated in synchronization with the time segments by anelectronically controlled electromagnetic coil or piezo-electric crystalstack on one edge of the mirror. The other edge of the mirror is hinged.Alternatively, reflection from counter-rotating mirrors is used tostabilize the projected image during a given time segment but to shiftit for the next time segment. The mirror may also be oscillated with afluid- or gelfilled piezo-electric prism (see FIG. 33) with two faceswhich are flat and rigid and hinged along one edge. The other threesides of the prism are collapsible. A stack of piezo-electric crystals3300 inside the prism causes it to change its angle in an oscillatingfashion in synchronization with an oscillating current.

The net result in either event will be to shift the image on the screenby one pixel for the second time segment and by another pixel for thethird time segment. Each screen pixel will therefore contain red, greenand blue information over time, giving the viewer a full-color imagewith no discernible color pixels anywhere, using a single, lowresolution light valve. It should be obvious that other arrangements canbe used to accomplish the same ends. This technique creates theperception of three times the resolution of the light valve, or theequivalent of three light valves.

Dead spaces between pixels will be visible whether a "full-color" lightvalve or multiple "mono-color" light valves are used, especially withthe use of an "active matrix." Although such an image may be acceptablein some cases, a better solution is to have all pixels superimposedexactly in triads (red, green and blue together forming "full-colorpixels") with spacing between such pixel triads eliminated, creating a"continuous image." In FIG. 4, each pixel 401 is a superposition of acorresponding red, blue and green pixel. 402 represents spaces whichneed to be filled. The following are methods to eliminate these deadspaces between pixels in the projected image.

The preferred method of elimination of spaces between full-color pixels(such as are created by the superimposition of the images of three lightvalves) uses lenses. A lens array 801 (as shown in FIGS. 8a and 8b)constructed with the same number of lenses as there are "full-color"pixels (e.g., the number of color "triads" on the light values arrangedwith the center of each lens over each pixel 802) could be used tomagnify each pixel as depicted in FIGS. 8a and 8b. Then optionallyeither a collimating lens array 803 as depicted in FIG. 8a or a largecollimating optic 804 as depicted in FIG. 8b could be used torecollimate the now enlarged and contiguous pixels for projection bysuitable projection optics.

If the spacing between pixels along the vertical is different than alongthe horizontal dimension, the pixels can be intentionally underfilledwith light, forming a symmetrical dot (as explained below) or anamorphiclenses or equivalent could be used to fill the spaces properly. Althoughfabrication of small lens arrays is within the state of the art, it issimpler and less expensive to use more readily available lenticularlenses. These cylindrical lens arrays can be overlapped with their axesperpendicular to one another to accomplish the same goal. The separationof lens function for each orthogonal dimension eliminates the need foranamorphic lenses which are difficult to produce accurately andconsistently in such small sizes.

It is important to note that eliminating the space between pixelsutilizing lenses after the pixels (and before the projection lens) canbe done with several different approaches. The lenslet curvature andspacing from the light valve can be selected to produce a real orvirtual magnified image of the pixel. These real or virtual images canbe magnified just the right amount so that they become contiguous at aplane in space. This plane is then imaged onto the screen by theprojection lens.

In actual practice, many virtual and real images of the pixels exist atvarious locations in spaces of different sizes. The projection lens canbe accordingly adjusted slightly back or forth to select the pixel imagesize which just eliminates the inter-pixel spaces without overlap.

If an arrangement is chosen (as described below) in which the source isimaged into each pixel hole, then the distribution of light within apixel may not be uniform. If it isn't, a repetitive structure will beapparent on the screen, making pixels visible, even if there actuallyare no spaces between pixels. In that event, the projection lens shouldnot focus an image of the pixel plane or a magnified real or virtualimage of its pixels onto the screen. Instead the projection lens canfocus an image of the lens array onto the screen. Each lenslet will beuniformly illuminated even if the light distribution within a pixelisn't uniform.

If the lens arrays aren't constructed well enough so that spacingbetween lenslets approaches zero, a pixel structure would again beapparent. To eliminate that problem, a second lens array could be usedto generate a magnified real or virtual image of the lenslets of thefirst array. Thus the "pixels" would appear uniform and be contiguous.

With a rear projection system built into a cabinet in which therelationship between the projector and the screen will never be altered,it is possible to build in a system to eliminate the space between thepixels right before the screen. A lens array with the same arrangementas the pixels projected from the projector, placed just behind thescreen, will expand the image of each pixel just enough to fill thespace between the pixels. This lens array can be built into the screenmaking it a rigid component of the screen.

Sometimes displays are made from a mosaic of smaller displays. Forinstance, CRTs are assembled in a matrix forming a "video wall" and thevideo image is segmented electronically so that each monitor displaysonly a part of the image, with the entire matrix of CRTs togetherdisplaying the entire image. Since CRT monitors can only be put soclose, there are noticeable spaces between them, creating a disjointedimage. This type of display can be depixellated as well, with theindividual monitors being considered as the "pixels". The variousconcepts contained herein can be used here as well. For instance, anarray of Fresnel lenses can be placed between the CRT array (with onelens for each CRT) and a rear screen. The CRT images are projected withslight magnification so that the resulting image appears seamless. Thiscan, of course, be applied to a mosaic of LCDs, multiple projectedimages, etc.

The following is a method for inexpensively making the lens arraysnecessary for the elimination of the spaces between pixels as well asfor other aims which involves creating a master for making lens arrays.The master can be made by taking a semi-soft material such as copper orwax and scoring it with parallel lines with a tool which has a circularcurvature at its end. A spherical lens array master can be made byforming a tool with a surface matching the lens surface desired andrepeatedly pressing it into such a soft material in a "step-and-repeat"fashion. This master can then be made into a hard metal master. If themaster is made in copper, the copper can be immersed in anelectroplating bath, such as nickel sulfamate. If the master is made ina non-conductive material such as wax, it can first be coated with athin metallic layer of electroless nickel or by spraying with astannous-chloride silver solution. Once metalized in this fashion, itcan then be placed into the electroplating bath. The nickel master canthen be placed on an embossing machine and used to emboss replicas intothermoplastic materials, such as mylar and plexiglass. Such a master canalso be used as a mold for injection or compression molding.

Another method of producing the master is to use a computer to make aplot in which the height of the lens is represented as a density. Thisplot, turned into a transparency, can be photo-reduced and replicated bystep-and-repeat procedures to produce a mask with a density patternwhich matches the lens array layout. The mask can then be imaged withultraviolet light onto a photoresist plate. The differing densities onthe mask will alter the amount that the photoresist is exposed and afterdevelopment, will alter the amount of photoresist that will be washedaway at each location. This will create a photoresist master in theshape of the lens array. This photoresist master can then by metalizedand used for replication.

An alternative method to produce such lens arrays for a projectionsystem is to use lens arrays produced holographically. Such holographiclenses are easier to produce than conventional lens machining at suchsmall dimensions, especially if extremely small F numbers are required.State-of-the-art methods can be used to create the necessaryinterference patterns.

As was done earlier to eliminate the appearance of red, green and bluepixels, a wedge or wedges may be used to create offset images on thescreen, both vertically and horizontally to eliminate the spaces betweenpixels. The wedge or wedge segments may be conveniently placed at theprojection lens to fill each space in the image with a duplicate of theadjacent image data, creating a focused, de-pixellated image. Thismethod is an alternate preferred method of eliminating spaces betweenpixels in the image.

Since the spaces between pixels are all horizontal and vertical lines ofa fixed width, spatial filtering may be used to eliminate the spaces.The classic method of spatial filtering is demonstrated in FIG. 32. Inthe input, image A is acted upon by lens 3310, creating a Fouriertransform in plane B. Another lens 3320, placed a focal length afterplane B, creates a Fourier transform of that transform which is theoriginal image in plane C. If a particular optical filter is placed inplane B, various components of the final image will be eliminated due tothe blockage in plane B of the Fourier components. The Fouriercomponents are arranged in a polar coordinate fashion in plane B withthe highest spatial frequencies which correspond to the smallestfeatures in the original image located throughout and towards theoutside of the Fourier plane. The low spatial frequencies in the imageare represented in the central area of the Fourier transform in plane B.Periodic input patterns are represented as localized concentrations ofintensities at that frequency in the Fourier plane. Since the thin linesrepresenting the spaces between pixels are high in spatial frequency,they will form large features, located mostly away from the center ofthe Fourier transform. Therefore, if an appropriate filter is placed inplane B, letting through the lower spatial frequencies, theretransformed image in plane C will have greatly diminished, or, if thefilter is selected properly, eliminated higher spatial frequencies(corresponding to the lines between pixels).

Since all pixels have the same spatial frequency in a given direction,which is different from the higher spatial frequency of the linesbetween them, those lines can be separated out and suppressed. The imageplane A is analogous to the light valve plane in the projector and thelens performing the Fourier transform is analogous to the projectionlens. Somewhere in front of the projection lens will therefore be anapproximation of a Fourier transform of the image on the light valve.Even though no second lens is used to re-transform the image after acertain distance, a re-transform will occur anyway (at the focused imageon the screen), making a final lens unnecessary. All that is necessaryin actual operation is therefore the placement of an appropriate filtersomewhere after the projection lens. Since the spatial frequency of theline pattern is known, state-of-the-art methods can be used to form aFourier filter to block out the desired spatial frequency components.The larger the difference between the width of the pixels and the widthof the spaces between pixels, the more efficient this spatial filteringprocess will be. As the widths approach each other, the process willbecome less effective.

Alternatively, if a lens is placed between the light valve and theprojection lens, the light can be made to come to a small focus withinthe projector. A pinhole can be placed at the focus, allowing most ofthe light to pass through. Passage of light through a re-transforminglens also placed before the projection lens will create a focused imagein space minus the high spatial frequencies of the image from the lightvalve plane. If the projection lens is then made to focus on that image,most of the light can be projected onto the screen without lines betweenthe pixels.

Another method of obtaining a brighter image is to use a holographicphase filter beyond the projection lens, constructed in ways that areknown in the state-of-the-art either with varying thickness material ora hologram properly laid out. This will still accomplish spatialfiltering but will allow more of the light to pass through to thescreen.

An alternate method of filling the spaces between pixels is by the useof mirrors. To make a mirror system that duplicates the pixels in theproper places with minimum waste of light, a special "striped-mirrorsystem" can be used. One such configuration is shown in FIG. 5. Lightcontaining full-color image information 501 (laid out as indicated inFIG. 4) hits a "striped-mirror pair" labeled as 502 and 503. This causesthe entire image to be duplicated and shifted horizontally the width ofone pixel with approximately one-half the brightness of the originalimage (which is also reduced to one half of its original brightness),filling the spaces between pixels in the horizontal rows as shown byFIG. 6. Vertical rows 601A, 602A, and 603A are duplications of verticalrows 601, 602, and 603, respectively. The combined (original andduplicated) image existing in space 504 of FIG. 5 then passes through asecond "striped-mirror pair" 505 and 506, which duplicates the image butshifts it vertically the height of one pixel. This produces two imagesof equal brightness, one above the other, filling in the horizontal rowsindicated in FIG. 6 as 610, 611, and 612. Thus, a "solid" image iscreated with no blank spaces. Elimination of blank or dead spaces,separately colored pixels, and thus the distinction between pixels,subjectively improves image resolution even above today's CRT images atclose range since CRTs have discernible lines, pixels and spaces.

A "striped-mirror pair" is better understood by reviewing FIG. 7. Lightfrom a single pixel 701 impinges upon a "clear" space 720 on the firstmirror 702 of the mirror pair. This first mirror is made of glass,plastic or other suitable material which is AR coated over the visiblespectrum and coated on its opposite side in stripes of a suitablereflective material such as aluminum or silver. The striped coating maybe accomplished by, for instance, vacuum deposition with a "striped maskover the glass." Alternatively, the glass can be coated withphoto-resist and exposed to a projected image of stripes of the desiredsize. After development, the glass will be exposed for metal vacuumdeposition only in the desired stripes. After deposition, the remainingresist could be peeled off or dissolved away, leaving the required clearstripes.

The second mirror 703 of the pair also has alternating clear andreflective stripes. On this mirror however, the reflective coating isthinner, creating partial mirrors instead of full mirrors. Thepercentage of reflectivity is adjusted so that the two pixel imageswhich emerge are of equal brightness.

Light from pixel 701, after passing through space 720, impinges onpartial mirror 730, creating a transmitted beam 710 and a reflected beamwhich hits mirrored surface 740 on first mirror 702. This reflects lightthrough clear space 750 on mirror 703 creating a second beam 710a whichis an exact duplicate of beam 710, except that it is contiguouslydisplaced from beam 710. If the spacing between pixels is not equal tothe dimensions of a pixel, the mirrored areas 740 on mirror 702, as wellas clear spaces 750 on mirror 703, may be adjusted to the dimensions ofthe space between pixels.

The overhead view of FIG. 5 shows that "striped-mirror pair" 502, 503,which has vertical stripes, is tilted with reference to beam 501 arounda "vertical tilt axis" to create a horizontally displaced duplicateimage and a "striped-mirror pair" 505, 506, which has horizontalstripes, tilted around a "horizontal tilt axis" (which is perpendicularto the tilt axis of the first "striped-mirror pair" and to the beam 501)to create a vertically displaced duplicate image.

Arrangements which break up a white collimated beam into coloredcollimated beams, as well as configurations which combine multi-coloredcollimated beams into a single collimated white beam are reversible andcan be used on either side of a light valve to make full use of alllight in the beam, illuminate a monochromatic light valve with theproperly colored beams, recombine the colored beams to form full-colorimages without individually discernible color pixels and produce animage which is continuous, having no spaces between the pixels.

The use of time multiplexing, as previously explained, can be used tofill dead spaces between pixels with duplicate pixels to create a"continuous" image. The three color images can be slightly offset tosomewhat fill the spaces between pixels. FIG. 3, for example, shows bluepixel 301 slightly higher than red pixel 302 and green pixel 303slightly to the left of each red pixel 302. Many other arrangements ofoffsets of the different colored pixels are possible to decrease blackspaces in the image; however, the individual colors remain visible atclose range.

To produce a good quality color image, it is important to have as high aresolution as possible, as well as to superimpose red, green and bluepixels on one another to eliminate the appearance of individual colorpixels and to eliminate spaces between pixels. Whether accomplishingthis with three optical paths and three light valves or by dividing up asingle light valve with a large number of pixels into 3 sections toproduce the 3 color images, the cost is higher and the system consumesmore space and weight than a simple single light valve system. However,a single light valve doesn't have the resolution of three light valves.It is therefore desirable to devise methods which produce a highquality, high resolution image without the added cost, complexity,weight and size increase as stated above.

Obviously, increasing the number of pixels in a light valve willincrease the resolution of the image. Two or more projectors used toproject contiguous images can produce an image with higher resolutionthan can be produced by a single projector using available light valves.Alternatively, a single projector can be made which essentially containsthe components of several projectors but with the contiguous imagesproduced side-by-side within the projector so that the composite imagecan be projected with a single projection lens. This will eliminate theneed for alignment of externally placed projectors and will produce ahigher resolution than is capable of being produced by a single lightvalve system.

Regardless of the relationship between the lines of pixels with respectto the placement of color dots within them, if any grouping of threecolored pixels of the light valve is used to form a color triadrepresenting the color of a particular point of the scene displayed,then the resolution capability of the LCD is reduced by a factor of 3.This resolution limitation can be reduced, however, if each pixel of thelight valve, being either red, green, or blue, is driven by a signalthat corresponds to that light valve pixel's color at that point in theoriginal scene, and the data about the remaining two color values atthat point in the original scene is simply discarded. The eye will tendto blend the color contributions of neighboring pixels to produce thecorrect color for that area of the scene, but retain the capability todistinguish detail as fine as the actual pixel spacing.

"Time-share scanning" (described herein) can be applied to create a highresolution image with a lower resolution light valve. For instance, animage can be projected having a space between every two pixels, alongeach horizontal line equal to the width of a pixel. This can beaccomplished, for instance, by fabricating the light valve that way orby using lenslet arrays to appropriately change the size of each pixel.Thus, if a light valve is capable of, for instance, 500 pixels on ahorizontal line, the resolution can be doubled to 1000 by time-sharescanning. One-half of the time can be used to project an image from thelight valve as it exists onto the screen, while the other half of thetime can be used to project an image of intermediate pixels onto thescreen, giving the image twice the resolution of the light valve in thatdirection. Unlike other time multiplexing schemes, no decrease inbrightness is created since each segment of time projects all of thelight from the light source onto the screen and thus all of the lightfrom the light source is visible to the viewer at all times. Thistechnique could also be used to double resolution in the verticaldirection creating, for instance, a high definition image from astandard resolution light valve.

The systems disclosed in this application can use discrete andindividually addressed and maintained pixels. This approach provides thebasis for true digital television. Presently both audio and videosignals are digitized and stored as digital bits on laser disks and"CDs." This digitization preserves the exact values of the signal frommicro-second to micro-second. Distortions in the systems, such asamplifier noise and non-linearity, scratches, dropout and other defectson the recording material and so on can be completely ignored by asystem looking only at each bit to see if it is on or off, i.e., a "0"off or a "1," and not caring if it varies in strength or clarity. Thiswill result in more precise, higher quality television and videodisplay. The upcoming thrust toward High Definition Television shouldmove the field toward this type of a digital display device as thesystem of choice. In summary, the present invention makes possible aviable basis for implementation of digital and High Definition TV,regardless of the format convention selected.

Use of digital processing makes it easy to eliminate the problemsinherent in today's video systems such as ghosts, chroma crawl, moirepatterns, snow and cross-talk between chrominance and luminance signals.It also makes the creation of additional pixels in the receiver byinterpolation between any two pixels possible, thus creating theappearance of even higher resolution in the receiver than is actuallytransmitted. It also makes special features very easy to implement suchas picture in picture, zooming, frame freezing, image enhancement,special effects and so on.

All electronic image production systems, whose images are made of afinite number of pixels, have an artifact which becomes more noticeableas the number of pixels in the image decreases. This artifact is oftenreferred to as jaggies or aliasing. When a diagonal line, such as aboundary between two different features, is presented in the image, theline becomes jagged, as if it were a staircase, since the pixels areusually square with their edges parallel and perpendicular to thehorizon. To reduce the noticeability of these jagged boundaries, knownanti-aliasing techniques can be implemented, especially if used in adigital system, since it is already computerized. When a boundary isdetected between two areas of different brightness values and/ordifferent color values, a calculation can be performed to find theaverage brightness and color between the two values. Then, making allpixels along the boundary that new value will create a transitionbetween the boundaries that is much harder to see, thereby reducing theappearance of a jagged edge.

The image brightness which can be produced by a projection system is inpart dependent on the bulb brightness. This generally means that formore brightness, a higher wattage bulb should be used. The bulb wattagethat can be used in many environments is, however, limited. A homeprojector shouldn't draw more than about 5 amps which corresponds toabout 600 watts. A higher wattage projector becomes very expensive tooperate and discharges a great deal of heat. It is therefore desirableto use a bulb which has as high an efficiency, measured in lumensproduced per watt consumed, as possible. The best light source uses amicrowave stimulated plasma. This type of bulb, currently in prototypeform, can produce up to 130 lumens per watt. Other sources which can beused include Xe, Hg and metal halide bulbs which can produce from 75 to95 lumens per watt. Tungsten halogen bulbs can produce as much as 40lumens per watt and regular tungsten can produce up to 25 lumens perwatt.

Instead of using a high powered bulb with a large filament or arc, twoor more bulbs of lower wattage and smaller filaments or arcs can beused. Using multiple lamps presents several advantages. If a lamp shouldburn out, the system would only diminish in brightness, operating withthe remaining lamp(s) until the lamp is replaced. Each bulb, being oflower wattage, can have a much longer lifetime, and a smaller filamentor arc can make focusing an image of the source into the pixel holeeasier. Various methods can be used to combine the beams for use. FIG.37 illustrates one example in which two light sources are collimated andmade contiguous by the use of a prism. FIG. 45 shows how a mirror can beused to make 2 collimated beams contiguous. Another method ofeliminating space between separate beams is the use of mirrors to takelight from one part of a beam and use it to fill in spaces betweenbeams. An example of this is illustrated in FIG. 38.

As can be seen from the figure, the light from the two sources can berearranged in this way to form what appears to be one Gaussian beam.Obviously, a different number of mirrors can be used and light deviatorsother than mirrors, such as prisms, can also be used. This technique canalso be utilized on several beams at once or on a single beam to alterbeam profile characteristics, such as rendering a Gaussian or anirregular beam profile more uniform. This is especially useful to getrid of weak or dead spots, holes, or hot spots in a beam.

Other examples of set-ups to accomplish this are depicted in FIG. 77. Inthis diagram, it is assumed that a beam is used that has a hole in themiddle, such as might be formed by an axial source which radiates atplus or minus, for instance, 60 degrees to the normal of the opticalaxis at the arc or filament. Parts A and B of FIG. 77 show alternatearrangements in which flat or curved annular mirrors 7710 (the mirrorsin A work by total internal reflection) reflect light form the outerportion of the beam to a central conical or axiconical reflectiveelement. This light then reflects forward, filling the hole in the beam.The intensity of the light filling the hole can be adjusted by alteringthe size of the annular mirrors, the slope angle of the conicalreflector, the curvature (if any) of the annular and conical surfaces(which imparts a lens function to the light), and light divergence orconvergence.

Alternatively, the beams can be made to come to a focus at an area inspace so that the filament or arc images abut one another, forming a newcomposite light source. By the use of mirrors, these point sources canbe made to propagate in the same direction, making it easy to collectwith a single condenser lens to form a single collimated light beamcontaining most of the light originally captured. An example of this isshown in FIG. 43.

Normally, combining two beams produces a beam that is either thecombined width of the two beams or one that has the combined divergenceof the two beams (or a combination of the two), as required by theLaGrange invariant. However, two beams can be combined into one beamwith no change in beam width or beam divergence, with the resulting beambeing either randomly or linearly polarized as desired, by using one ofthe following arrangements.

FIG. 83 shows how the light from two sources can be combined to produceone linearly polarized beam. 8300 and 8310 are two independent randomlypolarized white light sources. 8320 and 8325 are spherical reflectors.8330 and 8335 are collimating lenses. 8340 and 8345 are quarter-waveplates. 8350 and 8355 are MacNeille polarizers. 8360 is a first-surfacemirror. Output beam 8370 is linearly polarized. An alternate embodimentis depicted in FIG. 83B with only one MacNeille polarizer.

In FIG. 84, the arrangement is the same as in FIG. 83 except for theaddition of first-surface mirror 8465 and the reversal in orientation ofthe MacNeille polarizer 8455. In this case, the output beam 8470 israndomly polarized. An alternate embodiment is depicted in FIG. 84Busing only one MacNeille polarizer.

These set-ups can be cascaded several times allowing for the combinationof beams from a large number of sources, producing a single randomly orlinearly polarized beam with the same width and divergence as the beamfrom a single source.

The accuracy of reproduced color depends on several factors. With theuse of properly selected color filters or dichroic mirrors, correctionfor wavelength versus light valve cavity thickness versus voltage, asdescribed above, and normal Gamma correction and other normal TV colorcircuitry, the fidelity of color reproduction is still limited by thecolor makeup (i.e., color temperature) of the light passing through theprojection system. Incandescent lighting, although simple andinexpensive, produces a low color temperature, resulting in a "reddened"image, while discharge lamps, such as metal halide, xenon, mercury andespecially microwave driven plasma (which provides constant brightnessand color temperature even with tens of thousands of hours of operation)produce higher color temperature with more realistic whites and colors.However these lamps have the drawbacks of being more expensive, havebigger and heavier power supplies and are often more difficult anddangerous to use and replace. Realistic colors can be produced with theuse of incandescent sources if a color-temperature-compensating filteris used. At the expense of some brightness, the entire color spectrumcan be shifted towards the blue, producing more realistic whites andcolors. The advantages of using an incandescent source are that they arerugged, inexpensive, safe and easy to replace and need a small powersupply or no power supply at all.

A number of approaches might be taken to extend the life of the lightsource. The microwave stimulated plasma bulb for example has virtuallyan unlimited lifetime, and is thus best for eliminating bulbreplacement.

To extend the life of a filament bulb, circuitry could be used to runthe filament on smoothed DC. Furthermore, the circuit could ramp up thevoltage slowly whenever the lamp is turned on to reduce shock due torapid heating and filament motion.

For an incandescent bulb to have the highest efficiency as well as highcolor temperature, it is necessary for it to have a tightly woundfilament which runs on relatively low voltage and high amperage. Thiswould normally necessitate the use of a large and heavy step-downtransformer. To eliminate this burden, a triac circuit can be used tochop up the duty cycle, utilizing only part of each cycle. Selecting theproper duty cycle will provide the filament with the reduced voltagethat it requires. A feedback circuit can also be included to monitorline voltage and to adjust the duty cycle to compensate for line voltagechanges so that a constant reduced voltage is fed to the filament.

The projection systems described herein have brightness limitations dueto low efficiency at various points in the system. Various methods canbe used to increase the efficiency at these points and thereby theoverall efficiency and brightness of the projector can be dramaticallyincreased.

AR coating of all optical components significantly increases throughputsince approximately 4% is lost without it at every surface. Since thereare many surfaces in a video projection system, a significant amount oflight can be lost if this is not used. However, AR coating is done in avacuum chamber, making the process expensive and time consuming, with alimitation of how many pieces can be coated at a time. In addition, somecomponents can't be easily coated at all, such as LCDs. To remedy theselimitations, sheets or a roll of material such as mylar, cronar,polyester, or other clear material can be AR coated for later use. Whenneeded, such material can be attached to all optical surfaces with, forinstance, an index-matching pressure-sensitive adhesive. Since such amaterial can be easily cut, it can be made to conform to oddly shapedand angled surfaces.

Another way to provide an AR coating without a vacuum, is with the useof holography. By controlling the angles between interfering beams of atleast 90 degrees, a multi-layer interference pattern, resembling avacuum-coated multi-layer dielectric coating, can be produced quickly.Such holograms will thus act to suppress reflections and can be adheredto surfaces of optical components. Alternatively, holographic emulsions,such as photopolymers which require simple, non-wet processing, can becoated directly onto optical components and then exposed and processed.

One problem common to all projection systems is the efficiency of thelight collection optics. Usually, only a small percentage of the lightproduced by a bulb is actually collected and utilized in the projectionsystem. To further improve the efficiency of the system, various methodscan be used to increase the amount of light that is captured from thebulb for use in projection. In the prior art, a light source, such as afilament or arc, is positioned with a condenser lens, such as anaspheric condenser, in front of the source with a spherical mirrorbehind it. This arrangement is used in most projectors and captures someof the rearward and forward propagating light. The majority of thelight, however, propagates to the sides, upwards and downwards and iswasted.

A preferred method of utilizing this normally wasted light is the use ofmultiple condenser paths as shown in FIG. 42. Two condenser lenses 4210and 4220 and two spherical mirrors 4230 and 4240 will capture twice asmuch light emanating from a bulb 4200 as in the conventional system. Inall bulbs today, light traveling in one direction can never be utilizedsince one side of the bulb is used to connect power into the bulb to thearc or filament. Light from the remaining (upwards) direction can becaptured by an additional condenser lens 4250 and reflected by a mirror4260 into the system. The beams can be joined into a single beam usingthe methods described elsewhere herein.

Another method to utilize this otherwise wasted light is to place asection of a parabolic reflector 3910 around the lamp 3900 in aconventional condenser set-up 3920 as shown in FIG. 39. Light that wouldotherwise be unused is now collimated and sent forward to join lightemerging from the condenser lens. To reduce the size of the resultingcollimated beam, which will probably be necessary in most applications,various optical methods may be used, such as the Keplerian telescopemade of two lenses, as depicted in FIG. 40.

Another method used to capture more light from a bulb is depicted inFIG. 41. In this arrangement, a source 4100 is placed at one focus of anelliptical mirror 4110. Any light which hits this reflector will befocused to the second focus of the ellipse where it can be captured forcollimation, for instance, by a condenser lens 4120 with a low F number.However, light which misses the reflector (4101 and 4102), except forlight on axis, is lost. This light can be utilized by placing acollimating lens 4130 at the second focus. This lens will collimatelight that would miss the second focus, but will have almost no effecton the light going to the second focus. Lens 4150 can be used to bringnear-on-axis light to focus at the second focus of the ellipse forcollimation by lens 4120. The left-most portion of the ellipse can bereplaced by a spherical mirror 4140 for better overall systemcollimation.

Alternatively, a section of a parabola 4160 can be used to capture andcollimate that otherwise lost light. Lens 4150 and optional sphericalmirror 4140 behave the same way here as they do in FIG. 41. This can beseen in FIG. 46.

An alternate method of using an elliptical surface efficiently isdepicted in FIG. 50. In this set-up a spherical mirror 5010 makesrearward going light into forward going light. A lens 5020 capturesforward going light and brings it to a focus. A surrounding ellipticalsurface 5030 captures light which misses both the spherical reflectorand the focusing lens and brings it to the focus of the focusing lens.At this point light can be gathered from the focal point and collimatedby a single lens 5040.

Collection systems which capture light from wide angles, such as thosedisclosed herein, generally have large apertures. This leads to a largecollimated beam. As pointed out herein, such a beam can be reduced indiameter, for instance, by a telescope arrangement where the output lenshas a shorter focal length than the input lens. This reduction of beamdiameter is accomplished with an increase of angles of non-collimatedrays within the beam. This results in a restriction of how long theinternal optical path of the projection system can be before lightspreads so much that it doesn't get into the projection lens.

Several measures can be taken to condition the light to allow for anincreased internal path length if one is desired for a particular systemdesign.

A preferred method of dealing with this limitation is depicted in FIG.51. This method is accomplished by generating a reflector surface whichwill be referred to herein as a Fresnel Parabolic Reflector. (The samelogic can be used to produce other surfaces such as a Fresnel EllipticalReflector and so on.)

By assembling segments of a parabola (dashed curve), an equivalentparabola 5110 can be constructed with a narrow opening (solid need notbus, the collimated beam need not be reduced much, if at all. Thereby,angles are not increased and collimation length is left longer.

An alternate approach to this limitation is to use the idea used infiber optic cables. In such a cable, light can travel a long distancebut, because of continued low/loss internal reflections, the beamdiameter does not increase until the end of the "tunnel," which in oursystem can be where the light valve is placed. Light tunnels can be usedto obtain several important advantages in display systems such asobtaining more uniform illumination of the image forming element and theshaping of the light beam to conform to the shape of the IFE. Multipletunnels can be used if multiple light valves are used. Such a tunnel canbe made of mirrored surfaces instead of fibers and can take variousshapes such as square, rectangular or circular.

The tunnel can also be expanding or contracting, so that input andoutput beams can be different sizes with different divergences. Solidmaterials, such as glass and plastic, will perform as a mirror tunnel byutilizing total internal reflection. A mirror tunnel can be the samesize, bigger, or smaller than the image-forming element, and as long asdesired, and still deliver a major portion of the light to theimage-forming element placed after the output of the tunnel. As thetunnel gets smaller in cross-section or longer, the number ofreflections much of the light undergoes before leaving the tunnelincreases. Unless the surfaces are highly reflective (such as 95-99%),significant light could be lost. An advantage to multiple reflections isthat the light becomes more homogeneous or "scrambled" at the output ofthe tunnel due to the multiple reflections. This results in a moreuniform beam at the output of the tunnel than at the input, which isespecially useful when a beam has "holes", hot spots, or othernon-uniformities. A long tunnel can be "folded" by the use ofreflectors, prisms, or other re-directing elements between tunnels toproduce a more compact system, while the overall tunnel length can belong. The image-forming element can be placed at the output of thetunnel or, alternatively, the output of the tunnel can be focussed ontothe image-forming element putting this more uniform illumination at theimage-forming element. Further, matching the shape of the tunnel, andespecially its output aperture shape (these do not both have to be thesame) to that of the image-forming element can cause the light to fillthe image-forming element. This approach will minimize "spillover loss",which is found in all conventional projection systems in which acircular beam illuminates a rectangular aperture. This results in majorgain in system efficiency and, thus, brightness.

Placing a field lens just before the image-forming element reduces thedivergence angles (and, thus, increases collimation) of the lightilluminating the image-forming element. This is shown in FIGS. 67A andB. In 67A, light 6710 coming from the top of the tunnel 6720 illuminatesthe image-forming element (IFE) 6750 such that the central ray 6730 ofthe light bundle 6710 makes an angle θ with the optical axis of the IFE.The most extreme ray 6740 makes an angle φ with this central ray 6730,making the most extreme ray that hits the IFE an angle of θ+φ with theoptical axis of the IFE. Using a field lens 6760 as shown in 67B, bendslight bundle 6710 (as well as all others) so that its central ray 6730becomes parallel with the optical axis of the IFE. Now the largest angleany light ray makes with the optical axis of the IFE is just φ.

This results in brightness uniformity across the entire image since allrays illuminating the IFE, no matter which lens array element they gothrough, will illuminate the array element with the same cone angle withrespect to the normal to the IFE. Thus, all light gets through eachpixel hole, regardless of where on the IFE it is, since each lens arrayelement is illuminated with a light cone that falls within itsacceptance angle.

The non-imaging concentrator optics can be used to further reduce thebeam diameter, essentially allowing for the optical reduction of thesize of the light source. This will allow for the use of a brighterbulb, with a larger arc or filament. The concentrator optics, normallyused to concentrate light for solar collectors, can concentrate thelight to a smaller area than the original arc or filament. This willallow for greater collimation and, thus, permit more light into a longerpath system. One name commonly used to describe such a concentrator is a"compound parabolic concentrator" although the reflective surfaceactually has hyperbolic walls. The two currently known designs fornon-imaging concentrators, originating in the 1960s, are referred to as"edge-ray" concentrators and "geometric vector-flux" concentrators.

Such a concentrator can also be made with sides that are flat in oneplane. As an example, a concentrator with four sides that are flat inone plane is shown in FIG. 68.

A concentrator can also be used to produce a uniform collimated beam. Asthe concentrator expands, the light beam expands due to reflection froma sloping surface, decreasing its divergence and increasing itscollimation. Although this can also be done with a circularly symmetricconcentrator, using one with sides that are flat in one dimensionproduces a beam with a profile that matches the aspect ratio of the IFE.This is shown in FIG. 68 where a source 6800 has a spherical reflector6810 behind it to reimage the rearward going light as a second lightsource image formed at or near the actual arc or filament of the bulb.Large spherical mirror 6820 images these sources into the mouth of theconcentrator 6830. After the concentrator, lens 6840 helps increase thecollimation of the light which then illuminates the IFE 6850, which mayhave input lens array(s) in front of it.

To further increase the amount of light that gets into the projectionlens and thus, reaches the screen, the distance from the light valve(s)to the projection lens must be kept to a minimum (so non-collimatedlight gets into the projection lens). To accomplish this the focallength and F number of the projection lens should be kept to a minimum.

Light efficiency of projection systems in general is strongly affectedby the degree of collimation of the light in the system. The lesscollimated the light, the less of it that can make it all the waythrough the system. This is especially true the more elements there arein the system. The brightness enhancement techniques listed herein, suchas input lens arrays, dielectric polarizers and color dispersingelements such as holograms, binary optics, or prisms, becomeincreasingly ineffective as collimation decreases. Consequently, to takeadvantage of these brightness enhancement techniques and make thebrightest and most efficient projector possible, a sufficient degree ofcollimation must be present in the light within the projector.

Properly colored laser sources could be used as the source in such aprojector for the highest degree of collimation, since efficient diodelasers with high power have become available. Speckle can be cancelledby known methods such as a rotating phase plate or stationarymultifrequency phase plate.

Unfortunately, to make an incoherent source brighter, after choosing ahigh conversion efficiency material (such as metal halide) and aftermaking the arc (or filament, if one is used) as compact as possible, theonly thing that can be done is to make the source draw a higher wattage,which necessitates increasing overall arc or filament size. However, thelarger the source size, the less the light can be collimated, and theless of it that can get through the optical system. This would appear toput an upper limit on how bright a given system can be, regardless ofwattage.

A further increase in brightness can be achieved, however, by optimizinglight collimation and a system's ability to use it in the followingunique way.

The smaller a source appears to a collimating optical element (such as alens or a parabolic reflector) the more collimated the light will be.This means putting the source further from the collimating optic, usinga larger optic, and thus producing a larger collimated beam. Thus, anysource, whatever its size, can be highly collimated to virtually anydivergence/convergence tolerance, at the expense of space and the makingof a larger beam. At first analysis, this would seem useless as soon asthe collimated beam size exceeds the size of the light valve or otherimage-forming element to be illuminated. However, this excess light canbe utilized if it is re-directed, while still maintaining collimation,into the image-forming element with an element such as a prism (orprisms). This gets more light through the system at the expense ofcreating a lower F number requirement for the projection lens, which inmany cases is acceptable and worth the tradeoff to create a muchbrighter image.

For example, consider a system which uses a pixelbased LCD as theimage-forming element. Current active matrix LCDs have aperture ratiosof only 25%-45%. A typical pixel size is 60 microns on a side with apitch of 120 microns. Using an input lens array, as described elsewhereherein, will help only with highly collimated light. Typical glassthickness in an active matrix LCD is 1.1 mm. Considering an index ofrefraction of about 1.53, this gives an acceptance angle of about 5°,the acceptance angle being an angle at which light falling on the imageforming element will pass through the transparent apertures of thepixels rather than being blocked by the opaque pixel borders. (Lighthitting a lens element at 10°, for instance, will be mainly focussedonto an opaque area of the LCD, which can cause more local heating anddamage than if no lens was present. Assuming a 4" diagonal LCD with anaperture ratio of 25%, a typical metal halide arc of about 150 watts,which would be about 4 or 5 mm long, would be collimated by a reflectorof similar size to a divergence of about 10°. Mainly due to the Gaussiandistribution of the energy in the beam, placing a larger proportion ofthe light energy within the acceptance angle of the input lens array,some enhancement (perhaps up to a 50% increase) in light throughputcould be achieved by using an input lens array.

However, using a larger, (longer focal length) reflector, having, e.g.8-9 inches in diameter, would bring collimation up such that most of thelight would be concentrated within a 5° divergence. Virtually all of thelight that fell directly on the LCD would be focussed through the pixelholes by the input lens array.

Using prisms around the periphery of the beam, bending it back to theLCD, can get virtually all the light into the aperture window of theLCD. Even though this light enters the lens elements at angles above the5° acceptance angle, the light will get through for the followingreason.

The distance between the lens array and the pixel holes (1.1 mm in glassor 0.7 mm air equivalent) is much larger than the pixel spacing(typically around 100 microns or less). Thus, there are several anglesat which the LCD can be illuminated which will have approximately a 5°acceptance angle. Light focussed by a lenslet (after hitting the lensletat the right angle), instead of entering the pixel hole behind it,enters the next pixel over. FIG. 55 shows the intensity of transmittedlight through an LCD with an input lens array as the LCD is rotated on avertical axis. Utilizing a typical LCD, rotating through 84° provides 7transmission peaks (approximately every 12°), each with approximately a5° acceptance angle.

The larger the filament used, the greater the beam must be expanded toprovide the equivalent degree of collimation. A second set of prisms canbe used to divert light outside a 9" circle to the LCD at a higherangle, corresponding to the next transmission peak angle. Thus, in thisexample, light can be expanded to a circle with a diameter of up to 15inches, for instance, with all light still getting through an LCD thatis only 3 inches wide. Any larger size would most probably beimpractical because the F number of the projection lens would be lowerthan practicable (and the projector would become objectionably large andexpensive). A larger LCD and/or one with thinner walls would make iteasier to get more light through.

In a typical video LCD, every other horizontal row is offset from theprevious row by the width of one and one half pixels to provide a morerandom looking pixel pattern. This means that the peaks obtained byrotating the LCD about a horizontal axis are further apart than thoseobtained by rotating about a vertical axis. If the horizontal pixelpitch equals the vertical pixel pitch, the peaks would be twice as farapart rotating about a horizontal axis as they are rotating about avertical axis.

Light coming toward the LCD from above and below the LCD could bediverted up or down by two pixel rows (twice the angle) or it caninstead be diverted up or down to the next row and then diverted with asecond prism horizontally half a pixel. Various configurations areobviously possible. Thus, an input lens array element may focus lightinto a pixel hole which is located horizontally or vertically next tothe pixel directly in line with the input lens array element, or into apixel hole which is displaced both vertically or horizontally from thepixel hole directly in line with the input lens array element by one ormore pixels. The illuminated, by the input lens array element, pixelhole can be vertically displaced by one pixel and horizontally displacedby one and one-half pixels from directly in line pixel hole. Theilluminated pixel can also be displaced by two pixels vertically andthree pixels horizontally or by one pixel vertically and one-half pixelhorizontally from the directly in line pixel.

In addition to making most light in a projection system usable,especially in conjunction with other brightness enhancement techniquesthat work best with collimated light, this method has the following twoother major advantages.

First, light source--collector combinations often create non-uniformillumination, with intensity variations across the image plane as wellas color variations being visible. This technique superimposes severalbeams (usually nine) from different parts of the source on top of oneanother, allowing for better color and brightness mixing, and thus, moreuniform illumination. Prism angles, and distances between prisms and theimage-forming element can be altered to shift hot spots and more evenlyfill in "holes" to optimize resulting illumination uniformity.

Secondly, illumination systems typically gather light from a source andprovide a circular beam. Most image sources, on the other hand, producerectangular pictures. To fully illuminate the image in a conventionalprojector, the rectangular image must be circumscribed by the circle oflight such that the diameter of the circle must be at least as large asthe diagonal of the rectangle. This can result in the loss of up toapproximately 40% of the light. The technique just described folds allthe light into the image without that loss. Again, proper choice ofcollimated beam diameter, prism angles, and distance of prisms to imagesource can provide the most even illumination. To allow the light from acircular beam to more evenly illuminate a rectangular aperture, a pairof cylindrical lenses can be used to more closely match the aspect ratioof the image-forming element. FIG. 57 depicts an example of how thedifferent parts of a circular beam can be overlaid onto a rectangularimage to achieve efficient and uniform illumination.

Using the highly efficient composite collector depicted in FIG. 39 togather the majority of light coming from a uniformly radiating source,an example of this configuration is shown in top view in FIG. 58A andside view in FIG. 58B. In both figures the projection lens, locatedbeyond the image-forming element, is not shown.

Fresnel prisms can be used instead of standard prisms to save space,weight and cost.

In another embodiment, light can be gathered with an ellipticalreflector, especially if the source radiates little or no light forwardand rearward (as in a typical axially oriented arc), bringing the lightto a focus. A hot mirror can be placed somewhat near the focus to filterout much of the IR, while not requiring one that is too large. A coldmirror can be used to reflect the light while further reducing heatbeing sent to the light valve. Light spreading from this focus can becollimated, once sufficiently enlarged, by a Fresnel lens. Top and sideviews of this arrangement are shown in FIGS. 59A and 59B. FIGS. 60A and60B show plots of light intensity in the X and Y directions of a samplesystem on the screen obtained with this arrangement. Here a source whichwould otherwise have a "hole" in the middle of the image (if a parabolawere used) creates a uniformly illuminated screen.

A projector using this arrangement tends to be larger (e.g.,10"×10"×24") due to the distances needed for light to spread outsufficiently and for separate beams to be sent to the image-formingelement and overlapped at the proper angles. Various methods can beutilized to reduce these dimension requirements.

For example, several small arc (or filament)--low wattage bulbs withcollectors (6110) can be properly placed with a collimating lens (6120)adjacent to the image-forming element (6130) as shown in FIG. 61. Thiswill produce various collimated beams (6140) at the proper angles toenter adjacent pixels.

Alternatively, a larger single source (6210) can be sufficientlyexpanded to produce the required collimation, once collimated by a largelens 6270, after which different parts of the collimated beam can bebrought to foci (6250) which are, likewise, properly placed (by mirrors,prisms, etc.) to produce various collimated beams (6240) at the properangles once they pass through a collimating lens (6220) near theimage-forming element (6230). Such an arrangement is depicted in FIG.62.

In a preferred variation of this arrangement, each beam can be imaged byan intermediate focussing lens or focussing lenses, one for each beam.FIG. 65 shows the addition of focusing lenses 6560. Each lens 6560focuses an image of a portion (with the same shape as the IFE) ofcollimating lens 6570 onto the IFE 6530. The image can be made to fillpart or all of the IFE. As mentioned elsewhere, cylinder lenses can beused to match the light to the aspect ratio of the IFE. This methodprovides even illumination of the IFE 6530 with no spillover light,while still illuminating the IFE 6530 at the proper angles to befocussed by input lens array(s) 6580 into pixel holes (as explainedbelow). Light illuminating the IFE at the properly selected angles thatare off axis, will enter adjacent pixel holes as explained herein.

In a further preferred variation, as depicted in FIG. 66, light from asource 6600 enters light tunnel 6610. Lens 6620 focuses an image of theoutput of the tunnel (which has the same shape as the IFE 6660) into theplane of an array of lenses 6630 (which may be Fresnel lenses). Fresnelprisms 6640 bend different portions of the image towards the IFE 6660.Focussing lenses 6650 form images of the different sections of the imageat 6630 into the IFE 6660 illuminating part or all of it. The lenses inthe image plane 6630 focus the lens 6620 into the imaging lenses 6650 byway of the prisms 6640. The center section of 6630 needs no prism at6640. By expanding the light from the tunnel to a large area at 6630,divergence is decreased and collimation is increased by themagnification factor. This ultimately reduces the required distancebetween plane 6630 and the IFE 6660 while providing the collimationnecessary to fall within the acceptance angle of the lens array(s) 6670placed before the IFE 6600.

Alternatively, lens 6620 can focus the output of the tunnel 6610 ontothe IFE 6660 directly (eliminating 6630, 6640, and 6650). Knowing theoutput angle of the light emanating from the tunnel and knowing theacceptance angle of the input lens array (explained elsewhere, herein)determines the required magnification factor. Then, considering the sizeof the IFE, using that magnification factor in reverse determines therequired size of the output exit of the tunnel.

In another arrangement (see FIG. 63), to reduce overall projectionwidth, appropriately placed mirrors (6300) can be placed around all foursides of the optical system (6310), to allow the light to spreadsufficiently to produce the required collimation while still beingcollimated by a collimating lens (6320) at the proper angles toilluminate the image-forming element (6330). This is depicted with themirrors on two sides only for illustration purposes in FIG. 63.

If three light paths are used because three light valves are used tomodulate the red, green and blue images separately, the colored imagesmust be recombined to form a full-color image. This can be done withvarious arrangements, such as the one depicted in FIG. 2. However, tominimize the distance between the light valves and the projection lens,a dichroic combiner cube will keep the distances to a minimum. Such acube, known in the art, consists of four equilateral triangular prismsplaced together to form a cube. The faces that touch one another includedichroic coatings to allow the three colored image-bearing beams tocombine into a full-color image.

Conventional direct-view light valves utilize color filters to create afull-color image. Color filters work by absorption, which unfortunatelywastes approximately two-thirds of the light, converting it to heat,which exacerbates the heating problem.

A preferred method of making a color Image-forming Element (IFE)utilizes the deposition of dichroic mirror coatings on the IFE or on asubstrate adjacent to it. When deposited on an adjacent substrate(forming a dichroic filer plate), said dichroic filter plate can be usedwith an IFE alone, or in conjunction with an IFE that utilizes colorfilters. The latter arrangement relaxes the requirements for narrowbandwidth transmissivity of the dichroic coatings. The differentlycolored coatings can be deposited in a mosaic just as absorptive colorfilters are. This is depicted in FIG. 76. Just before the IFE is aseries of striped mirrors. Each mirror's width is twice the horizontalpixel pitch. The space between any two adjancent mirrors is equal to thehorizontal pixel pitch. Two lenticular lens arrays transform incominglight into a series of parallel lines of light. The width of each lineof light is equal to the horizontal pitch of the pixels of the IFE,while the space between any two adjacent lines of light is equal to twotimes the width of a line of light. Light illuminates the dichroicmirrors at a slight angle (dependant on the distance of the stripedmirrors from the dichroic mirror coatings and on the horizontal pixelpitch).

Any given line of light passes through the space between two-mirrors,illuminating a vertical column of pixels and half pixels (if every othercolumn of pixels is staggered, such as is depicted in FIG. 15B), or justilluminating whole pixels (if pixels are arranged as depicted in FIG.15A).

For a clarified understanding, consider that portion of the light thathits a single red pixel. Red light passes through the red dichroiccoating 7610 while blue and green light reflects back up to mirror 7620.After reflection from this mirror, these beams illuminate the greenpixel 7630, which is the next one over from the red pixel. The greenlight passes through the dichroic coating, while reflecting the bluelight once again for a final reflection from the stripe mirror. Thisblue light passes through the blue dichroic coating and the blue pixel7640.

An alternative method to making such a color mosaic without the use ofabsorptive color filters is illustrated in the following embodiment.FIG. 25 shows a collimated beam of white light 2500 which is separatedinto three collimated beams, one red 2510, one green 2520 and one blue2530, by a dichroic mirror arrangement 2540. These beams then passthrough a double lens array 2550, each array containing the same numberof lenses as the number of pixels in the light valve 2560. Each lenspair formed by one lens from each lens array produces a Galileantelescope, producing a collimated beam of reduced diameter. The lenscurvatures are chosen so as to provide a 3:1 reduction in diameter ofeach collimated beam. A second dichroic mirror arrangement 2570 bringsthe color beams together, but, due to displacement of two of themirrors, the beams do not actually overlap, forming a mosaic of colorsto illuminate the monochromatic light valve in whatever colorarrangement is chosen (such as the two arrangements described above anddepicted in FIGS. 15A and 15B).

An alternative method of producing a mosaic of colored beams isillustrated in FIG. 26. Collimated light 2600 passes through a doublelens array 2610, which again contains the same number of lenslets perarray as there are pixels in the light valve 2620. The focal lengths ofthe two arrays are different, such that a series of collimated beams isformed 2630. The width of each beam is the size of a pixel and thespacing between collimated beams is equal to twice the pixel pitch. Eachcollimated beam intercepts a stack of 3 special mirrors.

These "mirrors" consist of mirrored areas, separated by clear spaceswhich are twice the size of the mirrored areas. The width of themirrored areas is chosen so that each collimated beam will exactly filleach mirrored area when hitting the mirror at 45 degrees to the normalof the mirrors. Tracing the path of a single collimated beam emergingfrom one of the lenslets, the beam passes through clear areas in thefirst two-mirrors 2640 and 2650 in the stack, hitting a dichroicmirrored surface on the third mirror 2660. This dichroic mirrortransmits the red light and reflects the blue and green light downward.This blue-green beam hits a dichroic mirrored area on the 2nd mirror,which reflects a collimated green beam in a direction parallel to thered beam, while transmitting the blue beam. The blue beam hits the firstmirror, which is a standard first surface mirror, so that the beam isparallel to the red and green beams. These red, green and blue beamsilluminate three pixels on the light valve, which is monochromatic, butis addressed with red, green and blue data, respectively. Alternately,the dichroic mirrors could be replaced with volume holograms toaccomplish the same result.

In another embodiment, shown in FIG. 27, one of the collimatedmini-beams 2700 (as described above) hits a hologram 2710 whichrefracts/diffracts the light, breaking it up into essentially red, greenand blue beams. A second hologram 2730 or series of prisms bends theoff-axis beams back on axis, so that parallel red, green and blue beamsare formed, which can then illuminate a full-color light valve 2720, aspreviously explained.

White light, before it illuminates the light valve or otherimage-forming element, can be "broken up" into differently colored beamsby passage through prisms, gratings, and/or standard, blazed, binary orother holograms. The differently colored beams will be bent intoslightly different directions. Cascading two (or more) such elements canbe done allowing the spectrally separated beams to be redirected so thatthey are generally parallel to the optical axis of the system, or suchthat the green component is generally parallel to the optical axis, withthe other components on either side of the green beam converging towardsor diverging away from the green beam, as desired. This can be used todirect the properly colored components of the illuminating beam to thecorrectly colored filters for increased saturation of the primarycolors, permitting a more accurate reproduction of a larger gamut ofcolors. Alternatively, the different colored beams could illuminatecorresponding pixels without the use of color filters.

In a preferred embodiment depicted in FIG. 71, a white light beam 7100passes through such an element 7110 (or more than one such element witha cumulative spreading effect of the different colors) to produce nearlycompletely superimposed beams wherein each color component travels at aslightly different angle. The image-forming element 7130 is covered witha lens array 7120 in which each lens array element 7125 has a verticalpitch equal to the vertical pitch of the pixels of the light valve (orother pixellated, image-forming element) and has a horizontal pitchthree times that of the horizontal pitch of the pixels (making one thirdas many lenslets in the horizontal direction as there are pixels in thehorizontal direction). This design assumes pixels arranged in a fashionsuch as depicted in FIG. 15A or 15B, in which each horizontal rowalternates in color: R, G, B, R, G, B, and so on. If alternate pixelrows are offset from one another, so too are the lens array elements soas to match. The differently colored beams, travelling at slightlydifferent angles to one another, are focused to different positions byeach lens element 7125, corresponding to the pixels with information ineach color. Thus, the red end of the spectrum is directed to redinformation-bearing pixels, and likewise, for the blue end of thespectrum and the green-containing middle of the spectrum. As mentioned,color filters, placed at the pixels, can further filter these beams foradded color saturation. Use of an additional lens array or arrays canserve to focus all light from each segment of the spectrum into eachpixel so that little or no light is lost. This is explained in greaterdetail later on herein. Alternatively, careful selection of dispersionangle and lens array element 7125 focal length and its spacing frompixels can be used to filter each color spectrum segment by selectivelyblocking certain wavelengths by having them intentionally fall on opaquespaces between pixels.

If desired, the system can be tailored so that the axis of the greenlight is parallel to the normal of the image-forming element. Toaccomplish this, if prisms are used, for instance, they can beconstructed from different materials having different Abbe number butsimilar indices of refraction, such as acrylic and polystyrene. Placingthe prisms in opposition shifts the green light on axis, while leavingthe colors dispersed. If gratings or holograms are used, they can bemade with differing fringe spacings.

In an alternate embodiment, as shown in FIG. 72, two such colordispersing elements 7210 and 7220, which are identical, are placed inopposition to each other so that the effect of the first one iscancelled by the action of the second one when they are placed back toback. By placing the elements a slight distance apart, however, separatecolors can be seen. In FIG. 72, each lens array element 7235 in lensarray 7230 (in which the lenses again have three times the pitch of thepixels in the horizontal direction) creates a focussed image of thesource. As each focused beam passes through the first color dispersingelement 7210, the focussed spot image of the source is spread into aspectral line of color at an angle to the axis of the incoming beam. Thesecond color dispersing element 7220 then re-directs the differentlycolored beams so that the central color (green) is parallel to theoptical axis of the system when they reach the plane of pixel holes7240. Again, a separate lens array or arrays, as described later onherein, can focus each spectrum segment into its respective pixel so nolight is wasted or, without such additional array(s), spaces betweenpixels can be used to block specific wavelengths, as desired.

Fresnel prisms can be used to reduce size, weight and cost. If ahologram or a grating is used, a phase grating would produce the highestefficiency. One of the first orders could be used while the other firstorder, the zero order, and higher orders are suppressed. Holograms andblazed gratings work best at a specific wavelength. Consequently, formaximum efficiency, three could be used, one peaked for each desiredwavelength (red, green, and blue).

Using the same lens array, another method of separating the light intocolor beams uses dichroic mirrors. An example of how this can be done isshown in FIG. 73. In this arrangement, a white beam of light 7300 isbroken up into three colored beams by dichroic mirrors 7310, 7320, and7330 (the last mirror 7330 could be a front-surface mirror or prisminstead of a dichroic mirror). These mirrors are placed such that theydirect their individually colored beams through each lens array element7345 in lens array 7340 so that they illuminate the proper pixels 7350containing corresponding color pixel information. Front-surface mirror7315 can be used, for instance, to properly redirect the beam reflectingfrom mirror 7310 towards the lens array 7340.

Use of a dichroic or holographic system to produce a mosaic of coloredbeams can be done in conjunction with a color filter mosaic as well.Since the light is properly colored before hitting the filters, lesswill be absorbed and selected saturated colors will result.

Light valve systems that utilize rotation of the plane of polarizedlight have a major loss of efficiency because, to rotate polarizedlight, the light valve must be illuminated with polarized light. Systemsin use today make polarized light by using sheet polarizers whichproduce polarized light (inefficiently) by absorbing all light exceptthat which is polarized in the desired direction. This wastes more thantwo-thirds of the light and causes the polarizer to heat up. In thelight valve systems in use today, the polarizers are mounted on thelight valve. Thus, when the polarizer heats up, the light valve heatsup, limiting the amount of light that can be sent through the system.

One solution to this light valve heating problem is to mount thepolarizers a sufficient distance away from the light valve and to coolthe polarizers directly.

A better solution which also alleviates the inefficiency of sheetpolarizers is to use a MacNeille prism for polarization. The MacNeilleprism makes use of the fact that light which hits a dielectric surfaceat an angle, such as Brewster's angle, splits into reflected andtransmitted beams which are somewhat orthogonally polarized. This effectcan be maximized by applying several layers of dielectric coatings, withalternating indices of refraction, such as by vacuum deposition, ontothe surface between two glass prisms, cemented together to form a cube.

When the cube is properly constructed, approximately 50% of the lightentering the cube is transmitted as P-polarized light and approximately50% of the light is reflected by the diagonal surface as S-polarizedlight. Since most sheet polarizers absorb between 65% and 75% of thelight that hits them, just utilizing one of the beams from this cubewill increase the amount of light available for the light valve and willgreatly diminish the light valve heating problem caused by sheetpolarizer heating due to absorption. Both beams can actually be used sothat very little light is wasted in the process of providing polarizedlight for use by the light valve.

Both beams could be used by employing mirrors which reflect one of thebeams emerging from the cube such that its plane of polarization isrotated when the two beams are joined as side-by-side parallel beams oflight. As shown in FIG. 44, S-polarized light reflected by the cube 4400is reflected downwards by a mirror 4410, rotating the plane ofpolarization of the light with respect to the horizon. A second mirrorshown in the diagram as mirror 4420 reflects this light in the directionof the P-polarized light emerging from the cube while maintaining itspolarization orientation. By positioning this mirror at the right angle,this beam will be reflected up to the height of the P-polarized beamemerging from the cube. This beam is then reflected forward by a mirroror as shown in the diagram, refracted forward by a prism 4430 forming asecond beam of light parallel to the other beam emerging from the cube,both in its direction of propagation as well as in its plane ofpolarization. Each beam can be brought to a focus with the use of lensesand mirror right next to each other, forming a single expandingpolarized light beam. Other methods described herein could also be usedto combine the beams so that both would illuminate the light valve.

A preferred method of utilizing both beams produced by a polarizationbeam splitter cube 5400 is depicted in FIG. 54. With this method, amirror 5410 which is parallel to the dielectrically coated diagonal ofthe cube is placed adjacent to the cube, producing two side-by-sidecollimated beams with orthogonal polarizations. Placing a half waveplate 5420 in one of the beams produces two side-by-side parallel beamswhich have the same polarization. The size and aspect ratio of theresulting beam can be altered by the use of spherical 5430 andcylindrical 5440 lenses, if required.

If a large beam must be polarized, using a MacNeille prism willunfortunately require a heavy, large, solid beam splitter cube which isexpensive to produce and consumes much space. A small beam of lightcould therefore be used, although this may require using additionallenses and additional space to accommodate the changes to the size ofthe beam. Unfortunately, reducing beam size increases the angles ofnon-collimated rays, which then polarize inefficiently in such a cube. AMacNeille plate polarizer which weighs less and consumes less space canbe used but will function only over a very narrow bandwidth. In a videoprojection system, as contemplated by the present invention, a beam ofwhite light could be separated into three color component beams by, forinstance, a dichroic mirror system. These three separate colorcomponents could then be sent to three MacNeille plate polarizers.Although this does save space and weight, the optics required toseparate and recombine the colored beams may occupy the same or agreater amount of space and weight than was saved. Moreover, the threeMacNeille beam splitter plates would greatly increase the cost of thesystem. Applicant has devised a "Fresnel MacNeille prism," whichfunctions as a MacNeille prism beam splitter but has, at the outersurfaces of the plates, a multiplicity Of tiny saw-tooth surfaces, eachbehaving as a normal prism. This device weighs much less than a prism,consumes less space, operates over the entire visible spectrum, andcosts less to produce.

FIG. 78 depicts this device. A multi-layer dielectric coating 7800 isdeposited on the flat surfaces of a saw-tooth component 7810 which ismade, preferably, of a plastic such as polycarbonate. Once coated,matching sawtooth component 7820 is glued to it forming a polarizingbeam splitter plate 7830. Collimated light 7840 illuminates the plate7830 at 45 degrees. Each corresponding saw-tooth pair (for instance,7850) acts as a MacNeille prism transmitting P-polarized light andreflecting S-polarized light in a perpendicular direction. Thisseparation of P- and S-polarized light takes place across the entirepolarizing beam splitter 7830. As described herein, the S-polarizedlight can, for instance, be reflected from a mirror 7860 and passedthrough a half-wave plate 7870 to become P-polarized light beforeproceeding to the IFE. As with a standard MacNeille polarizer cube,these two beams can remain side by side or overlap when illuminating theIFE.

This type of polarizer, like a MacNeille polarizer, eliminates the lossof light due to absorption and the heating of standard sheet polarizers.In addition, it eliminates the cost and weight of the prisms in aMacNeille polarizer, which become heavier and more expensive as the beamto be polarized increases in size.

The arrangement depicted in FIG. 54, comprising the beam splitter cube5400 and reflector 5410 can be miniaturized as well to reduce weight andcost. One arrangement to accomplish that is depicted in FIG. 64A. Byjoining a right angle prism 6430, to one of the prisms making up thebeam splitter cube 6410, a Rhomboid shape 6450 is formed which can bemade from a solid material such as polycarbonate. This Rhomboid isjoined to the other right angle prism of the cube after an appropriatedielectric coating is deposited on one of the surfaces between them at6420. 6440 is half-wave material as depicted in FIG. 54 as 5420. Severalof these units 6450 can be arranged, for instance, along the line whichis offset 22.5 degrees from the optical axis of an incoming light beam6400. These individual units 6450 could be held in place between flatplates 6460, which are held together by appropriate fasteners 6470.

The randomly polarized beam 6400 entering this array of polarizingprisms and reflectors will be reflected in a perpendicular direction andexit as a linearly polarized beam 6480.

This configuration can be modified for easy mass-production by injectionmolding, for instance, components 6435 and 6415. These components can beglued together after the appropriate dielectric coating is deposited oneither surface at interface 6425.

Randomly polarized light 6405 illuminating the composite structure 6455will be split into alternating beams of P- and S- polarization 6465travelling perpendicular to their input direction 6405. After passagethrough half-wave material 6445 placed in all beams of the samepolarization, the emerging light beam will be linearly polarized all inone orientation.

If desired, this beam can be redirected so that it is propagatingparallel to the original beam direction 6405 with the original beamdiameter 6485. This can be accomplished, for instance, with the use of adouble lenticular lens array 6490 and a "Fresnel mirror" 6495, whichcould consist of mirrored surfaces or prisms which operate by totalinternal reflection.

Since the Fresnel polarizer plate 7830 of FIG. 78 must be illuminated at45 degrees, and the Fresnel polarizer plate 6455 of FIG. 64B must beilluminated at 22.5 degrees, they still take up the same amount of spaceas a MacNeille polarizing beam splitter. This space requirement can bealleviated by utilizing any one of a number of possible Fresnelpolarizer configurations. Some sample configurations are describedbelow.

A saw-tooth structure such as depicted in FIG. 79, made, for instance,of a plastic such as polycarbonate, used in conjunction with lenticularlenses will polarize white light even though it is illuminating thepolarizing structure at normal incidence. Collimated light 7900illuminates double lenticular lens 7910 (in which each positive-negativelens pair forms a Keplerian telescope) forming collimated beams of light7920, each of which is half the width of its corresponding lenticularlens element. Each beam passes through a portion of the first plasticelement 7950 which is flat on one side and contains 45-degree-anglesloping surfaces on its other side. Coated onto at least all of theslanted surfaces that slant upwards from left to right is a dielectriccoating 7960 of different index materials such as SI02 and TI02deposited in alternating layers, as is known in the art, to make aMacNeille polarizer. Glued to this coating is identical plasticcomponent 7970. Each said light beam 7920 passing through substrate 7950interacts with the coating 7960 such that all P- polarized light istransmitted to exit the flat face of component 7970 while all S-polarized light is reflected to the right where it is again reflected bycoating 7960 back towards the source. The light beam then passes throughquarter wave plate 7940, which changes it to circularly polarized light.Upon reflection from mirror 7930, the circularly polarized lightreverses handedness and, after again passing through quarter wave plate7940, becomes P-polarized light, allowing it to pass through coating7960 and element 7970. Thus beam 7980, emerging from element 7970consists of almost completely P-polarized light.

A variation of this arrangement is depicted in FIG. 80. In thisarrangement, components 8050 and 8070 consist of saw-tooth surfaceswhich have 45 degree sloping sides, each of which connects to a sidewhich is parallel to the optical axis. As in the previous example,lenticular lenses 8010 produce parallel beams 8020 whose width is halfthat of a lenticular lens element. Again the P-polarized light passesthrough the multi-layer coating 8060 and exits through 8070. However, inthis arrangement the S-polarized light reflects sideways to the nextslanted surface, reflecting once again from coating 8060 in a directionparallel to the optical axis. This S-polarized beam then passes throughhalf wave plate 8040 and becomes P-polarized light. Thus, beam 8080exiting this polarizer consists primarily of P-polarized light.

A preferred variation of this arrangement is depicted in FIG. 81. Inthis variation, component 8150 has half as many saw-teeth as 8170. Sinceall saw-teeth on both components are both the same size, this isaccomplished by spacing the saw-teeth on component 8150 twice as farapart as the saw-teeth on component 8170. After deposition of anappropriate coating on 8160, the two components 8150 and 8170 are gluedtogether as before. In this arrangement, P-polarized light emanatingfrom the lenticular lens passes straight through parallel to the opticaxis. S-polarized light reflects perpendicular to the axis from themulti-layer coating 8160 and passes into the adjacent saw-tooth from8170. The beam then reflects by total internal reflection parallel tothe optical axis. After passing through half-wave plate 8140, thisS-polarized light becomes P-polarized light. Thus, the final beam 8180consists primarily of P-polarized light.

A re-arrangement of the saw-teeth can be done so as to double the pitchof the lenticular lenses and the half-wave plate segments while leavingthe size of the saw-teeth teeth the same. This rearrangement is depictedin FIG. 82.

This can also be done using a variation of the system depicted in FIG.64. This is depicted in FIG. 85 in which the incoming light beam 8500 isreduced by double lenticular lens array 8510 to several parallel beams8520 which have a space between every two beams equal to the width ofone of the beams. These beams illuminate structure 8530 at normalincidence, which can be made, for instance, from two injection-moldedparts 8540 and 8550. These parts are glued together after being coatedwith the appropriate multi-layer coating on slanted surfaces 8560.Half-wave material 8570, placed in every other beam emerging from 8530produces a single linearly polarized beam 8580 which is the same sizeas, and is co-linear with, input beam 8500 (although it has twice thedivergence in one dimension as input beam 8500).

Other similar variations are possible and are within the scope of thepresent invention.

Another way to reduce the size, weight, and cost of the MacNeille orFresnel polarizer is with the use of holograms or simple diffractiongratings.

The prism necessary in a broad-band MacNeille or Fresnel polarizer takesincoming light and refracts it so that it impinges on the multi-layercoating at Brewster's angle. The output prism likewise takes the lightemerging from the multi-layer coating and refracts it so that it is onceagain parallel to the optical axis. If these prisms are replaced byappropriate holograms or gratings, the same functions can beaccomplished in a much smaller space (since the multi-layer coating canbe placed perpendicular to the optical axis) without the weight ofprisms.

All previously described MacNeille polarizers and Fresnel polarizershave utilized multi-layer dielectric coatings which must be applied withvacuum deposition. This is somewhat expensive and time consuming. Ahologram, which can be recorded with a single exposure, provides analternative to such a multi-layer coating at a lower cost in much lesstime. This can be accomplished by making a volume hologram in which theangle between the interfering beams is greater than 90 degrees. Thestanding-wave pattern set-up within the emulsion provides alternatinglayers of high and low indices with a single quick exposure. This"stack" is similar in form and function to the multi-layer stackconventionally created by vacuum deposition. Since holograms work mostefficiently at a given wavelength, performance may be optimized, whenusing white light, by superimposing several holograms (such as onepeaked in the red, one peaked in the green, and one peaked in the blue).Three separate finished holograms can be assembled together or threeseparate exposures can be made in the same emulsion, varying either thereference beam angle or the emulsion thickness with an agent such astriethanolamine between exposures.

Various arrangements, such as depicted in FIGS. 54, 64, 80, 81, 82 and85, for instance, can utilize a cholesteric-nematic liquid crystalinstead of a multi-layer dielectric coating. The unique anisotropicoptical properties of liquid crystals allow them to be used to splitunpolarized light into right- and left-handed circularly polarizedbeams. After reflection of one of the beams, both beams becomecircularly polarized with the same handedness. Then, passage through aquarter-wave plate converts these beams into linearly polarized light.Since cholesteric filters tend to be most efficient at a specificwavelength, a sandwich of several "tuned filters" (such as one for red,one for green, and one for blue) can be utilized to provide efficientpolarization of white light.

Any of these methods to produce a relatively flat "polarizer plate" or"Fresnel polarizer" that can be illuminated at normal incidence have usewhere sheet polarizers are currently used as well as places where sheetpolarizers aren't used because too little light passes through. One suchuse is for polarizing vehicle headlights and windshields inperpendicular axes. This dramatically reduces glare from oncomingheadlights, while allowing the majority of other light, including lightfrom a vehicle's own headlights once it has diffusely reflected from anyobject, to pass through the windshield to be seen by the driver.

Linearly polarized light that passes through an ordinary lens is nolonger strictly linearly polarized. This is because a lens consists ofcurved surfaces which can alter the polarization of light passingthrough it due to the dielectric polarization effect mentioned above. Asa lens surface is continually curving and changing its angle withrespect to different portions of the beam of light, different portionsof the beam's polarization are altered differently. This will reducecontrast and color fidelity of the image produced by a light valve usingpolarized light. To reduce this problem, if a polarizer is used, itshould be positioned after any lenses, whenever possible. The preferredsolution is to use lenses which are as thin as possible, even if severalare used in sequence, coated with highly efficient AR coatings on thecurved lens surfaces to minimize the polarization effects encounteredwhen light hits a surface at an angle.

Although a MacNeille polarization beam splitter allows approximately 50%of the input light to be transmitted as P-polarized light, each beam,specially the reflected S-polarized beam, is somewhat impure. In otherwords, the transmitted beam, although primarily P-polarized, containssome non-P-polarized light, while the reflected beam, although primarilyS-polarized, contains some non-S-polarized light. A small amount of such"contamination" is very noticeable to the eye, making the projection ofcompletely black areas impossible, reducing the contrast and colorsaturation. To solve this problem, a polarizer could be positionedbetween the MacNeille beam splitter and the light valve with their axisparallel, causing a relatively small loss of light, but eliminatinglight of the unwanted polarization, improving the contrast ratiopotential from approximately 20:1 to approximately 1000:1 and onlyincreasing the light loss from 13% to 35%, which leaves twice as muchlight as with the use of just a polarizer.

The use of a dichroic beam combiner cube to produce a full-color imagefrom three separately colored image-bearing beams within a small spacehas been explained above. The same cube can also be coated to operate asa MacNeille polarization beam combiner cube. This cube will act as abeam analyzer for light valves using polarized light. With thisarrangement, one beam will be transmitted through the cube, while theother two beams will be reflected by the internal surfaces.Consequently, the transmitted beam must be P-polarized while thereflected beams must be S-polarized. The light exiting the light valvewhich is to be transmitted by the cube must be P-polarized while theother two light valves must be manufactured to provide images inS-polarized light. Light polarized by the MacNeille methods disclosedherein, being all of one polarization, can be rotated by a half waveplate before entering the light valve which requires orthogonalpolarization. However, a simpler and less expensive alternative is theuse of identical light valves (as to required polarization) and ahalfwave plate after the light valve which produces a differentpolarization output from the other image-forming elements.

A major loss of efficiency which is especially noticeable in an activematrix light valve occurs because there are spaces between pixels whichdo not transmit light. Light that hits these areas does not reach thescreen, decreasing the brightness of the projected image andcontributing to heating of the light valve. Typically between 25% and45% of the light illuminating such a light valve actually passes throughit. To get around this problem, light must be crammed into the pixelholes, being made to miss the opaque areas between pixels.

The preferred technique to do this utilizes lenses to focus light comingfrom the condenser system down into the pixel holes. For a given lightvalve, the pixel hole size is fixed. Selecting a bulb fixes the filamentor arc size. To get as much light as possible from the selected lightsource into the pixel requires taking into account a few factors.

Although a transverse filament or arc source can be used, an axialfilament or arc within a reflector is preferred. There are severalreasons for this choice:

1. The closer the source is to the reflector, the poorer will be thecollimation of the light. An axial source stays farthest from thereflector, whereas a transverse source gets nearer to the reflector formost of the source.

2. An axial source radiates most of its light sideways to be reflectedby the reflector, with little or no light going toward the base of thereflector or forward. A transverse source radiates much of its light atthe base, which performs the most poorly of all areas on the reflector(in terms of collimation) due to its closeness to the source, andforward, missing the reflector entirely, and not benefiting, therefore,from the reflector's function.

3. The symmetry of the axial source within the reflector creates a muchmore symmetrical illumination of the image-forming element than atransverse source would provide.

4. A transverse source would have to be demagnified much more than anaxial source for its image to be focused by a lens array into the pixelholes. As the practical demagnification is limited, use of a transversesource, thus, further reduces the amount of light that can be focused bythe lens array into the pixel holes.

5. Although some of these problems are helped by using a sphericalback-reflector and a condenser lens, this doubles the transverse size ofthe source (worsening collimation) and loses the majority of light sincemost light misses both the reflector and the condenser lens.

The thickness of the glass used in the image-forming element limits howclosely the lens array can be to the pixel hole and thus how short thefocal length of the lens array can be. The ratio of the focal length ofthe condenser lens system to the lens array focal length determines thedemagnification of the filament or arc image. Although we would like alarge condenser focal length so that the demagnification factor issufficient to focus the entire image of the filament/arc into the pixel,increasing the condenser focal length decreases the amount of light itcan gather from the filament. Consequently, we must have the condenserfocal length as short as possible while still demagnifying the image ofthe filament/arc sufficiently to fit within the pixel (taking intoaccount diffraction blur). We must therefore select a bulb with thesmallest filament or arc size that will provide the minimum acceptablebrightness. With a given pixel size, a minimum lens array focal length,a given filament size, a maximum filament efficiency per unit area and aminimum condenser lens focal length, the maximum amount of light thatcan be put through the pixel holes is determined. Using theseparameters, a light source and lenses can be chosen to get as much lightthrough the light valve as possible for any given image-forming element.As disclosed earlier, techniques such as the use of a collimatinghologram or the use of non-imaging concentrator optics can reduce thefilament/arc size, allowing more light to be focused into the pixelholes.

Although the use of a single lens arrays before the image-formingelement can provide some increase in throughput, in a real system wherethe light is not truly collimated because the source has a finite size,some problems still remain, depending on the optical configuration used.One of the problems is non-uniform illumination causing the appearanceof structure within each pixel. To deal with this problem, one method isto use a Kohler-type illumination arrangement. With this type ofarrangement, the illuminated area will appear fairly uniform even thoughthe source may be non-uniform (such as with a filament). The input lenscan be considered as the condenser lens and the depixellating lens afterthe light valve can be considered to be the projection lens of thesystem. In this case, the light source image is focused into or near thedepixellating lens. This is shown in FIG. 34. The illumination at anypoint on the array after the image-forming element (used fordepixelization) is proportional to the brightness of the source and thesolid angle through which that point is illuminated. As seen in FIG. 34,the illumination angle 3410 from the center of the output lens array3420, positioned after the image-forming element 3430 to magnify theimages of the pixels and eliminate the spaces between the pixels in theimage, is that which is subtended by the array element 3440 placedbefore the image-forming element, assuming the pixel hole allows theentire cone of light to get through to the array element after theimage-forming element. When looking at the light which hits a point onthe lower edge 3450 of an array element after the image-forming element,as also shown in FIG. 34, we can see that the lower edge of the pixelhole limits the cone angle of light 3460 available to illuminate thearray element after the image-forming element. Thus, illumination alongthe edge of the array element after the image-forming element will peakat about 50% of the illumination at the center of the element and falloff to about 25% at the corner of the element.

With each pixel being brightest in its center and dimmer around itsedges, a pixel structure would still appear visible on the screen eventhough there was actually no space between pixels. This problem can becircumvented by the use of a second or field lens. As shown in FIG. 36,ideally the field lens array 3600 at the pixel plane would cause thelight that would miss the array 3610 after the light valve (due toblockage by the pixel apertures) to be redirected, resulting in uniformillumination as seen from the last lens array 3610. In reality however,the field lens array cannot be placed exactly in the pixel plane.Consequently, we can split the field lens array into two lens arrays,one on either side of the light valve, placed as close to the lightvalve as possible. With this arrangement, depicted in FIG. 56, the firstlens array 5610 focuses an image of the source with the help of thefirst field lens 5620 to an area 5630 beyond the pixel hole. The secondfield lens 5640 (being the first lens array after the light valve) helpssteer the light toward the final lens array 5650. This final arraymagnifies the image of the pixel forming an image to be projected on thescreen by the projection lens. This magnified image of the pixel, asexplained earlier, abuts the magnified image of its neighboring pixel,causing a continuous image made of contiguous pixels, with no spacesbetween them on the screen. It can be seen in the figure that the fourthlens element 5650 is actually not necessary and the third lens element5640 can be placed in one of many different locations after the lightvalve to accomplish depixellization. In this case, however, it can beseen that practically anywhere that the depixellating lens is placed(given the proper focal length) will still result in a uniformlyilluminated pixel without any vignetting caused by the pixel aperture.Thus the use of two input lens arrays instead of one improves pixelillumination uniformity.

The most severe problem in using a lens to squeeze the light through thepixel hole, which is not alleviated by the use of a single lens arraybefore the image-forming element, exists due to the thickness of theimage-forming element glass. FIG. 35 depicts the arrangement in whichthe light from the source is focused into the pixel hole.

If the illuminating light source were a true point source, depicted asthe center of the lamp filament 3500, light would focus as a result ofpassing through the array element 3510 before the light valve into thecenter of the pixel 3520 and then fully illuminate the array element3530 after the light valve. This would cause a complete uniformillumination of each pixel on the screen, and all of the source lightwould get through the pixel hole.

However, since the filament is extended and not a true point source,light will be entering the array element before the image-formingelement from other positions and at other angles. As seen in the Figure,light coming toward the image-forming element off axis 3540 will come toa focus in the pixel hole at 3550 right at the edge of the pixel hole.Any light approaching the image-forming element from a greater off axisangle will be focused onto the opaque area and not go through the pixelhole, defeating the purpose of using the input lens array.

As an alternative method to focusing light into the pixels, two lensarrays can be used as an array of Galilean or Keplerian telescopes. Withthis method, collimated light entering the first lens array element willlikewise enter the pixel as collimated light. However, since a realsource has a finite extent, collimated bundles of light will also enterthe first lens array at various off axis angles. This will, again, limitthe amount of light that can get through the pixel hole since light thatenters from too high an off axis angle will be directed onto the opaqueareas of the image-forming element and not get through.

Due to the glass thickness of typical active matrix LCDs, for instance,and the size of a typical pixel hole, the fastest light cone that couldbe produced by a lens array placed against the outside of the LCD wouldbe about F6. This F number can be reduced and the acceptance angle ofthe lens array element can be increased by using thinner glass or bycreating GRIN lenses within the glass used to form one of the sides ofthe LCD. Either of these methods would bring the lens closer to thepixel hole, allowing approximately a doubling of acceptance angle.Aberrations could limit the value of further decreases in F number.Short of using these methods, which require a new light valve design,and utilizing the LCDs that are available today, the preferred method ofincreasing the light throughput through the image-forming element isbased on using two input lens arrays wherein the first lens arraycreates an image of the light source in space the size of the pixelhole. Since there is no glass spacer in the way, an F2.5 lens can beused, doubling the acceptance angle of the system. The second lens arrayperforms a one to one imaging of that aerial image of the source intothe pixel hole, thereby making the thickness of the image-formingelement glass irrelevant. This is depicted in FIG. 69 where 6910 is thefirst input lens array element, 6920 is the aerial image of the source,6930 is the second input lens array, and 6940 is the pixel hole, withthe image formed at 6940 being the image of 6920.

Making the image of the source at 6920 as small as the pixel holeincreases the angles of light emanating from it so that the light isdirected toward multiple lens array elements in the second lens arrayand is thereby focused into multiple pixels. Light from a single aerialimage is, thus, directed in this arrangement to every other pixel (asshown). All light goes through pixel holes and none is focused ontospaces between pixels. In this case, the input angles of the light tothe first lens array must be controlled carefully, or else alternatingpixels will have a different brightness level than the remaining pixels.To reduce this dependence on careful control of the angles of inputlight, the lens array elements of the second lens array can be made thesame size as the pixel holes (doubling the number of lens array elementsin each direction) so that the light from the source image formed by thefirst input lens array element illuminates several lens array elementson the second array and the same number of pixel holes, without skippingany pixels. This is depicted in FIG. 70.

Variations are possible. For instance, the image of the source formed bythe first input lens array need not be the size of a pixel. In thatcase, the lens distances and the focal lengths can be changed to performother than 1:1 imaging of the aerial image of the source into the pixelhole.

Again, use of two input lens arrays has an advantage over the use of oneinput lens array. By doubling the acceptance angle of the input lensarray system, more light gets into the pixel holes of the IFE,considering that the light is not perfectly collimated.

If the filament image is not uniform, the distances can be adjusted sothat the second lens array element focuses an image of the uniformlyilluminated first lens array element into the pixel hole.

Alternatively, three lens arrays can be used so that the second lensarray element forms an aerial image of the first (uniformly illuminated)lens array element the size of a pixel hole. This aerial image is imagedinto the pixel hole by the third lens array element.

Light can be sent to the IFE with input lens array(s) as a result ofbeing focussed by an imaging lens, forming an image of the exit of alight tunnel. Use of a field lens just before the input lens array(s)bends each light bundle's principal rays so that they are parallel withthe optical axis of the system. The use of the field lens reduces theangles of light illuminating the IFE at all points. This allows for theuse of input lenses with the same pitch as the pixels.

Light can address the IFE such that its principal rays are not parallelto the optical axis of the system (it can be converging or diverging)and still get through the pixel holes via the lenses if the pitch of thelenses is adjusted to be larger (for converging light) or smaller (fordiverging light) than the pitch of the pixel holes.

In an arrangement using a light tunnel which is focussed into the planeof the IFE, for example, using a lens array with a smaller pitch thanthe pitch of the pixel holes (with the imaging lens being smaller thanthe IFE) replaces the need for a field lens near the IFE. This way, eachbundle of rays hitting any given point on the IFE has its principal raysparallel to the optical axis formed by each lens element and itscorresponding pixel hole. Thus, the angles of light illuminating eachlens array element are kept to a minimum, and the cone of lightilluminating each pixel is within the acceptance angle, allowing all ofit to go through each pixel hole, creating a uniformly illuminatedimage.

In a preferred embodiment of the system of the present invention, theinput lens array system, as just described, which can improve systemefficiency by a factor of about two (depending on the loss present dueto the IFE aperture ratio) could be used in conjunction with the methoddescribed earlier herein in which the light is separated into its colorsbefore being sent to the differently colored pixels. This technique willprovide an additional gain in system efficiency on the order of anotherfactor of two, making total possible system efficiency improvement ofapproximately 4 times by using both techniques together. To use themtogether, the optics must be configured so that light of the propercolors illuminate the proper pixels.

One way to accomplish this is depicted in FIG. 74 in which light hasbeen separated into three differently colored beams which illuminate theIFE at different angles. Two input lens arrays are used. The first lensarray pitch is three times the pixel pitch in the horizontal and equalto the pixel pitch in the vertical. The second lens array has twice thepitch of the pixels in the horizontal direction and the same pitch asthe pixels in the vertical. Other variations are obviously possible.

Lens array element 7415 of array 7410 creates three images of thesource, one for each color, at twice the size of a pixel hole. Lensarray 7420 images these sources into the pixel holes 7430 at2×demagnification. Because of the geometry, as can be seen in thefigure, a source image of any one given color is imaged into every thirdpixel, corresponding to data of that color. This produces a properlycolored image with a potential gain of four times.

If a method of breaking up the colors is used that produces separate,differently colored beams whose principal rays are parallel to eachother, then the first input lens array 7410, if necessary to form thecolored aerial images of the source, can have the same pitch as thesecond array 7420.

If a method for breaking up the colors is used that produces acontinuous spectrum instead of three discrete beams, one for each color,then an additional lens array can be used with a pitch equal to that ofarray 7420. This array, placed between arrays 7410 and 7420 would actsuch that each element would take one third of the spectrum produced byeach lens element 7415 in array 7410 and create an aerial image of itwith twice the width of a pixel hole. The 2:1 imaging of these aerialimages into the pixel holes by array 7420 would then take place asbefore.

The previous example assumed that color filters, if used, were in theplane of the pixels. If color filters are used that are outside of theplane of the pixels, a different arrangement may be required, dependingon where the color filters are placed. If the filters are placed nearthe colored aerial images of the source, the same set-up will workproperly. However, if the filters are placed just outside the IFE, and,consequently, near the final input array, a different arrangement mustbe used.

In that case, if light beams (white or colored) illuminate the IFEthrough such filters other than parallel to the axis of the system,their angles must be carefully chosen so that the proper colored lightbeams go through the corresponding color pixels. A beam of any givencolor that illuminates the IFE off axis will still contribute to aproperly colored image if it is either shifted vertically by two rows orif it is shifted horizontally by one and one-half pixels and verticallyby one pixel. Such shifting is easily done using known optical methodsor those described herein. This assumes a pixel arrangement such as isdepicted in FIG. 15b.

The same optical techniques can be used with a single input lens arrayas well. Various combinations of the methods described herein canobviously be utilized for combined benefits within a system.

As an example, multiple light sources can be used, with each onefocussed into a separate light tunnel. Each light tunnel's output canthen be focussed onto the same IFE, being careful to select the properoff-axis angle so that each off-axis beam enters the IFE one pixel overafter passing through the same lens array elements as the on-axis beam.

Once light has been crammed into the pixel holes, it most likely will beemerging from them as a diverging beam, diverging at a narrow angle suchas 5 degrees. While this is fine for projection, when these techniquesare used for direct view, the possible angles of view may be consideredtoo narrow. The angle of view can be substantially increased by the useof a lens array after the image-forming element (such as an LCD) whichfocuses an image of each pixel onto a rear screen. Such a screen,especially of the types disclosed herein with selectable gain and angleof view, will make the display evenly visible over as wide an angle asdesired. This technique is useful even if no techniques are used to cramlight into the pixels, as is the case in today's direct-view displays.Such displays, such as are found in a laptop computer, also suffer fromcolor shifting and loss of contrast when viewed from angles other thannormal to the display. These problems can be eliminated with thistechnique. A high-gain screen with high transmissivity will provide animage nearly as bright as that of the display when viewed directly, butwith a wider field of view. Even diffusely illuminated displays, such asLCDs, have a relatively narrow field of view which can be increased inthis way.

With the increased popularity of "letter-boxed" movies, which morenearly match the aspect ratios of movie theater projection and HDTV,another problem occurs which also can waste light.

Since light valves, for instance, are generally not totally opaque whenno signal is applied, a letter-boxed image will show leaked light in theareas above and below the picture where it should be perfectly dark. Toeliminate this light leak, opaque "shutters" can be brought into the topand bottom areas of the light valve, as close to the image plane aspossible, to assure no light gets to the screen in the areas that shouldbe black when the image area is smaller than the active area of thelight valve.

To eliminate the waste of light that occurs in this situation, a pair ofcylinder lenses or prisms can be used in the beam before the light valveto alter the aspect ratio of the beam so that all available lightilluminates the image-bearing area only.

With these methods, the higher the spatial coherence of the light source(the more of a "point-source" it is) the more efficiently these methodswill operate. However, to produce more light or to make a bulb with alonger life, requires the use of a larger lighted area. To takeadvantage of such sources, with the techniques described herein, thesource size must be reduced by "funneling" the light down to a smallpoint.

Another method of cramming light into the pixel holes is by using afiber optic bundle in which the input end is tightly packed and theoutput end is arranged so that each fiber is the same size as itsadjacent pixel hole.

There is one other source of wasted light in a video projection systemwhich is never thought of as wasted light. This is the light that isremoved from certain areas in the image because those areas are supposedto appear as darker areas. This is light that should not reach thescreen so that brightness variations can be produced on the screen tocreate an image. However, this light need not be totally lost.

With the use of a light valve that utilizes polarized light, a polarizeris used after the light valve to act as an analyzer. Light that shouldnot appear on the screen exits the light valve polarized perpendicularto the axis of this polarizer/analyzer and is thereby absorbed by thepolarizer. This generates some heat as well, which can heat up the lightvalve, if the polarizer is near it, and is also inefficient in that only25% to 35% of the light that should be going to the screen makes itthrough the polarizer/analyzer. By using a MacNeille polarization beamsplitter or a Fresnel polarizer (as described herein) instead of thefinal polarizer/analyzer, several advantages are realized. Since thereis no absorption, no heating occurs. Because nearly 50% of the lightappears in each beam, nearly 100% of the light that should go to thescreen passes through the analyzer to the screen. A plane mirror in thepath of the beam exiting the MacNeille analyzer that normally would havebeen absorbed by a sheet polarizer can reflect that normally wasted beamback to the light source for reprojection through the system to theextent the beam is collimated. The beam will retrace its path throughthe system ending up being focussed into the center of the light sourceto be gathered by the collecting mirrors for reprojection through thesystem. Although a large portion of this light will not make it to thescreen due to non-parallelism, and consequent inability to retrace itspath through the entire system, and due to loss of improperly polarizedlight exiting the first MacNeille polarization beam splitter or Fresnelpolarizer on its way back to the bulb, some brightness will be added tothe image that would not have been available if this technique were notused.

The various light saving or "brightness enhancement" techniquesdisclosed herein can greatly increase the light output of most displaysincluding projection systems and direct view systems such as LCDs.

As an example of how much improvement can be achieved, consider a singleLCD-type projector. Use of more efficient light collection can doublebrightness. Shaping the beam to fit-the LCD can result in over a 30%gain. Use of a non-absorbing polarizer that makes use of bothpolarizations can double the brightness. Splitting up the white light sothat the proper color light addresses the proper pixels so light is notabsorbed can more than double brightness. Use of light that has beencollimated sufficiently to match the acceptance angle of input lensarrays can more than double brightness, depending on the aperture ratioof the LCD. Since these enhancements are multiplicative, multiplyingthem together gives a theoretical brightness gain over a standard LCDprojection system of approximately 20 times or more than 2000%. Actuallyachievable gain in the real world is usually less than predicted bytheory, but nonetheless can be quite substantial.

Many projection formats can be used in conjunction with the disclosedvideo display systems. In addition to curved, direction-sensitive, highreflectance screens, less expensive, more widely dispersive screens canbe used with this system. A regular movie screen or even a wall provesadequate with a system of such high brightness. By vertical mounting ofthe unit or the attachment to the projection lens of a front-surfacemirror, the image can be displayed on a bedroom ceiling. This techniqueallows for convenient viewing of video imagery while lying in bed,without causing neck or back strain.

Rear-screen projection can be achieved as well. Conventional rear-screentelevision utilizes a lenticular lens and a Fresnel lens for adequatebrightness. This adds a discernible pattern to the image and produces alimited angle of viewing both horizontally and vertically. This type ofscreen, like a conventional CRT, reflects ambient light to the viewer,creating glare which adds to the viewer's eye strain. With the presentsystem, brightness is much higher, allowing for a broader viewing angleas well as more streamlined, lightweight and aesthetically pleasingdisplay units.

The high brightness allows for the use of a gray matte (i.e., textured)screen material with wide dispersion angles. This creates an image thatis viewable from practically any angle with uniform brightness and noglare. This type of glareless screen, coupled with the ability to varythe brightness and color temperature of the display by selection of bulbtype and operating voltage, may also provide a significantly lessfatiguing display for individuals who must spend long hours staring at avideo display terminal.

One of the most efficient types of screen (front or rear) can be madeusing holography. With a hologram, a diffuser can be produced with apredetermined dispersion pattern, creating as much diffusion as desired,with precisely tailored brightness distribution characteristics.Efficiency can approach 100% The interference pattern can be madeoptically for simple specifications or by computer generation for morecomplex characteristics. Bleached or gelatin phase holograms ormetalized embossed holograms can be used to produce the actual screenwith high efficiency.

With rear-screen projection, rather than locating the projector severalfeet behind the screen to allow the image to expand sufficiently to fillthe screen, one or more mirrors can be used to reflect the beam one ormore times to allow image expansion within a smaller cabinet size. Forinstance, a cabinet approximately 18" deep could be used to fill a rearprojection screen with a diagonal measurement of 50".

When an image projected on a screen is viewed in an environment wherethere is much ambient light, the areas of the screen that should be darkbecome filled with the ambient light, reducing contrast in the image. Atype of screen can be constructed which will provide a bright image withhigh contrast in high ambient light situations in both front and rearprojection. The front projection version of this screen is depicted inFIG. 47 and comprises a regular front projection screen such as abeaded, flat white or metallic coated screen. On top of the screen is ablack mask with relatively thin horizontal slits. A lenticular lenswhose cylindrical lenslets are oriented horizontally is placed on top ofthe slit mask. There is one slit for every cylindrical lenslet. Formaximum versatility, the slit mask is adjustable in the verticaldirection. Light from the projector focuses an image on the lenticularlens sheet of this screen, breaking the image into many horizontalsub-images corresponding to the number of horizontal cylindricallenslets. Each lenslet focuses its image component to a thin line whichpasses through the corresponding slit in the mask to be reflected fromthe screen behind it. This reflected light is re-expanded by thecylindrical lenslet for viewing with high visibility from all angles.Ambient light arriving at the screen from any height other than that ofthe projector (which makes up most ambient light), will be focussed bythe lenslets onto the black light absorbing layer and will not bevisible to the viewers.

The rear projection version of this screen is constructed by placing twohorizontally oriented lenticular lens sheets back-to-back with theirflat sides towards each other. The slit mask described above is placedbetween the lenticular lens sheets. Optionally a highly transmissiverear screen material can be placed next to the slit mask (also betweenthe lens sheets). The screen operates in the same manner as the frontprojection version to eliminate ambient light reaching the viewer. Inboth front and rear configurations, the slit mask can be adjusted up ordown to allow the light from the projector to pass exactly through theslits, depending on the projector's height in relation to the screen.

A rear-projection screen can be made with a selected gain by using alayer of micro-prisms on a substrate such as lucite. Microprisms can bemolecular in size by using transparent molecules which have non-parallelsides such as silicon or polymer molecules. Increasing the molecularconcentration and/or coating thickness will lower the gain and increasethe angle of view without noticeable brightness falloff. The best gain,in the applicant's opinion, is a compromise of perceived brightness whenviewing the screen head-on (where most viewing will be done) and thelargest viewing angle without a noticeable falloff in brightness.Experiment shows the best compromise at a gain of between 1.3 and 1.4.Charcoal or other dye molecules can be added to adjust perceived imagecolor temperature, to create a darker black to increase contrast, and toprovide absorption of ambient light. Carbon and dye added to such ascreen with a 1.3 gain so as to produce a transmissivity of 45-48% givesthe best compromise of transmitted image intensity loss verses contrastimprovement and ambient light absorption improvement.

A holographic screen can be made in a number of ways to accomplish thesame ends. For example, a hologram can be made of the screen justdescribed having a selected gain with the screen focused into the planeof the hologram by the use of a large lens or a second hologram (as isknown in the art). Using a reference beam at normal incidence, or theangle most likely to be taken by a projector illuminating the finishedscreen, with approximately a 1:1 beam ratio will result in a highertransmissivity than the original screen being holographed, especially ifa phase hologram is used (such as bleached silver, DCG, orphotopolymer).

Another way to make a holographic screen with a specified gain is to usea carefully illuminated hemispherical diffuser with selected brightnessat different angles as the "object" of the hologram.

A holographic screen can be made as a computer-generated hologram bymodeling the "object" just described and its interference with anappropriate reference.

Holographic screens have the advantage of being able to adjust the gainand angle of view, both horizontally and vertically, independently ofeach other.

Taking advantage of the fact that light from a light-valve is usuallypolarized, ambient light rejection can be increased in a screen withrelatively little effect on image brightness by placing a linearpolarizer on one or both surfaces of a screen. Only a few percent ofambient light which hits the screen will get to the viewer while thepolarizer whose axis matches that of the polarized light from the image,will attenuate the image relatively little.

Since polarizing material is conveniently made with a fixed axisorientation relative to the material roll length, when such polarizingmaterial is put on a screen, its axis may not match the axis of thepolarized beam illuminating it. This can be remedied inexpensively byplacing a sheet of half-wave material in the beam and rotating it formaximum screen brightness. The half-wave material can be placed in theprojector or at or near the projection lens where the beam is small,allowing for the use of a small piece of material.

Ambient light that is reflected specularly from the surface of thepolarizer will be reflected to the viewer without attenuation from thepolarizer. Although this usually represents a small portion of ambientlight, in some circumstances this will still be objectionable. Tosignificantly reduce such specular reflection, a thin material such aspolyester, mylar, or other thermo-plastic can be embossed with adiffuser pattern. This pattern can, for instance, be the surface of apiece of sandblasted glass or other non-planar surface. This materialcan be applied to the surface of the polarizer with an index-matchingpressure-sensitive adhesive.

Alternatively, the polarizer can be AR coated or adhered to AR materialas described elsewhere herein.

These methods are useful on both front and rear projection screens.

Another method could be used to reduce ambient light reflection. Thevideo projector's image can be focussed onto the input end of a coherentfiber optic bundle. This is shown in FIG. 17 as 1795 which places theinput end of the fiber bundle into the projected beam instead of screen1790. The other end of the fibers 1797 can be flat or polished intolenses or can be coupled to lenses. Thus each fiber, separated fromneighboring fibers, can magnify (due to fiber separation and due to thelens) and deliver to a rear-screen a portion of the image (preferablyone pixel or part of a pixel per fiber), magnified a predeterminedamount. The composite image will appear continuous, creating a verylarge image, with only a few inches of cabinet thickness since thefibers can bend. This technique also eliminates the need for any othersubsystem to fill the spaces between pixels. Using the fiber opticscreen with the fibers spread apart at the output end, no lenses, and noscreen, in conjunction with black, light absorbing material to fill thespaces between the fibers will produce a bright image in an area withhigh ambient light such as in an outdoor stadium. This is because amajority of the surface area of the output of the fiber bundle will beabsorptive to ambient light, while all of the image bearing light willstill be sent to the viewer. However this is done at the cost ofcreating a pixel-like structure due to the spaces introduced between thefibers. When viewing a large projected image in this situation however,the viewers are generally positioned at some distance from the screenwhich will make the pixel structure virtually invisible to the viewers.

An example of an artistic and futuristic projection system isillustrated by FIG. 14. The video projector 1401 can be mounted to anupright 1402 projecting an image onto a mirror 1403. Mirror 1403 canreflect the image to onto a special rear screen 1404 mounted in a framewhich appears to be "hanging in space." The screen itself can be made ofextremely thin slats 1405 of almost any rear projection material. Bymounting an axle onto the ends of each slat with a gear on each, a motordrive can be used to open (slats lying flat and parallel to the floor)and close the slats (lying perpendicular to the floor, creating a solidrear screen for projection). In the open position, the screen willappear as a transparent window in space. When the projection unit isturned on, by remote control for instance, the slats can simultaneouslyand quickly be closed, creating a "video image in space."

Whatever projection method is used, two other important problems canoccur. Unless the surface being projected upon is perpendicular to theoptical axis of the projection beam, the image will suffer fromkeystoning and blurring of the parts of the picture not preciselyfocussed on the screen surface. This problem is inherent if theprojector is mounted on the floor, on a low table, or on the ceilingwhile the screen is centered on a wall. CRT systems handle keystoning byvarying the electromagnetic scan line deflection. Some light-valve basedsystems, however, have predefined pixel locations and thus cannotutilize this technique.

Consequently a type of anamorphic lens system can be constructed. A zoomlens normally changes the size of a projected image by changing therelative positions between the elements of the projection optics.However this could also be accomplished if lens elements of differentcurvatures were used. Applicant's system could employ a lens which isshaped as if it has added to it two varying focal length lenses, oneabove and one below the standard lens molded into one lens. The centralarea of the lens, large enough to encompass the entire light beam fromthe valve, creates a rectangular projected image. But if this lens israised or lowered with respect to the light valve, the magnificationvaries across the image, causing a trapezoidal image predistortion witheither the top or bottom of the image of the light valve being thelargest side of the trapezoid. Thus, the lens is adjusted up or down,depending on the angle the video projector is making with the screen andthereby the keystone effect is cancelled.

The variable focus problem can be corrected by a little-knownphotographic technique known as "Scheimpflug correction." If a scene tobe photographed has a large depth and a fairly large aperture is used,the only way to simultaneously focus all elements of the scene is totilt the lens and film plane such that a line drawn through all objectsin the scene intersects the line drawn through the film plane at thesame point that it intersects a line drawn through the lens plane. In acamera, this is accomplished by bellows. Using the same logic, amechanical adjustment that tilts the light valve plane and the plane ofthe projection optics, creating an intersection with a line passingthrough the screen plane, will cause the entire image to be in focus,even though the projector's beam is not perpendicularly aimed at thescreen.

Science fiction has always portrayed the video display of the future asa thin large screen that hangs on the wall and modern day technologistshave been working towards that end for decades. With an image projectedonto a wall, the idea is almost realized. However, projection onto awall mandates that the projection distance be included as part of thesystem because nothing may be placed between the projection lens and thewall. Applicant has devised a new type of screen which would eliminatethis intervening space or projection distance. With this screen, theprojector can be placed underneath it or even be built into the screenitself, and yet the entire device thickness need not exceed a fewinches. This screen takes advantage of the phenomenon that a beam oflight of small diameter shone on a surface at a very oblique angle canbe spread over a huge distance. When the propagation direction of thelight beam is nearly parallel to a surface, the beam can illuminate theentire surface, even if the surface is hundreds of times larger than thediameter of the beam, with no projection distance necessary before thelight hits the surface. Spreading of a light beam by shining on anoblique surface "expands" the light beam's dimensions in one direction.If the surface could then re-direct the very wide beam, onto anothersurface, again at an oblique angle, but orthogonal to the first surface,the beam could again be spread in the orthogonal direction with noprojection distance required.

This re-direction is realized by a surface with saw-tooth shape elementswith the sloping side of each saw-tooth mirrored 4700, forming a"Fresnel mirror." As shown in FIG. 47, this will spread the light over alarge area, but will create horizontal stripes of light with darkhorizontal stripes between them 4710. The smaller these reflectors, themore of them there are, and the less noticeable the black bars in theimage. To make the light coverage continuous and eliminate the darkstripes, the sloping surface of each saw-tooth need only be curvedslightly to expand the segment of light that hits a given saw-toothsufficiently to cover half of the dark band on either side of the lightband reflected by the saw-tooth. Alternatively, a lenticular lens can beplaced between the saw-toothed surface and the imaging area.

An alternate method of producing a surface that will behave as requiredis to use known techniques to produce a holographic surface that willre-direct the light into the right directions.

If the light beam aimed at such a "Fresnel mirror," contains an image,the image will be spread in one direction onto the surface of theFresnel mirror. If the Fresnel mirror 4800 is placed at an oblique angleto a rear screen 4810, as is shown in FIG. 48, the image will now beexpanded in the orthogonal direction, filling the entire screen.However, since the image viewed from a rear screen appears brightestwhen looking at the screen towards the source illuminating the screen,the screen would be its brightest only when viewed at an oblique angle.Adding a second Fresnel mirror 4900 to re-direct the light in adirection normal to the screen 4910 makes the image visible on thescreen brightest when viewing in a normal fashion. (See FIG. 49.)

Alternatively, instead of utilizing curved saw-tooth surfaces orlenticular lenses after reflection from each Fresnel surface, aspherical lens array can be placed just before the final viewing screento eliminate spaces between sections of the image.

Two distortions are created by projecting onto a screen by way ofFresnel mirrors. Since the image spreads out in all directions as itpropagates, the image will be wider the further it has to go, with thefurthest end being wider than the nearest end. This trapezoidaldistortion will be repeated in the orthogonal direction when reflectingfrom the second Fresnel mirror. These two trapezoidal distortions can becorrected by pre-distorting the image trapezoidally in both axes withappropriate lenses in the opposite directions of the trapezoidaldistortions that will be encountered due to spreading. The seconddistortion is focus distortion due to the widely varying distance fromthe projection lens to the near part of the image versus the distancefrom the projection lens to the far part of the image. This focusdistortion can be corrected by tilting the projection lens with respectto the light valve plane in the direction opposite to the screen tilt.This tilt uses Scheimpflug correction (described above) so that theentire image is in focus on the screen, even though it is beingprojected at oblique angles. Such a screen system could be used for theprojection of any type of image, including slides and movies as well.

Although projection systems generally project their images on some sortof screen, in some instances it would be advantageous to projectdirectly onto the retina of one's eye. Since a light valve, such as anLCD, can be made very small and lightweight, and using some of thetechniques listed herein, an efficient projector can be made which isvery compact and lightweight. It then becomes feasible to mount such asystem on a headband or pair of glasses so as to give the viewer his ownprivate viewing screen. Because the entire retina can be projected upon,the viewer can see his entire field of view covered with the image. Ifthe image is projected into one eye only, the viewer will be able to seethe projected image all around him, but, it will appear superimposed onthe real world. This technique could be especially useful for privateviewing of a movie or confidential data, without others seeing it, orfor providing a computer screen to be connected to a computer in placeof a monitor. This application would free the viewer's body and headfrom being constrained to one position for long periods of time.

In place of a conventional projection lens or condenser system, compactoptics such as lens arrays can be used to image each pixel onto theretina with a corresponding lenslet for each pixel. Alternatively,compound holographic optical element could be used or multiple curvedreflectors facing each other's reflective surfaces, with on and off-axiselements to reflect and image a light valve onto the retina could beused.

The present invention lends itself to three-dimensional videoprojection. One method of accomplishing 3-D projection is to use twoprojection systems with the polarizers of one light valve systemperpendicular to the polarizers of the other light valve system. Sendingstereoscopic video signals, derived from two displaced cameras forinstance, and projecting onto a non-depolarizing screen will allowviewers wearing polarized glasses to see full-color 3-D video. A singlelens 3-D video projection system can be constructed by placing bothlight valve systems in one enclosure. Internally, the two orthogonallypolarized stereoscopic images can be joined by a MacNeille prism.Alternatively, instead of using the second mirror 503 of the first"striped mirror pair" 502 and 503 of FIG. 5, the horizontally displacedspaces between the pixels of one light valve can be filled by the pixelsof the other light valve through a simple beam splitter set-up, creatinga horizontally interlaced, orthogonally polarized 3-D image pair forprojection through the single projection lens. Striped mirror 502 can betilted at a 45 degree angle with respect to the axis of the light fromthe first light valve. The light from the pixels of this light valvewill pass through the clear areas of the striped mirror. The secondlight valve, whose axis is perpendicular to the axis of the first lightvalve, reflects its light from the mirrored areas of the striped mirror,causing an interlaced composite image made from both images, withorthogonal polarization.

Another method of 3-D projection which can be used is auto-stereoscopic3-D projection. This method does not require any special glasses for 3-Dviewing. Two identical lenticular lens screens, with their cylindersoriented vertically, placed back-to-back, optionally, with a thintranslucent screen between them are projected upon at different anglesby two or more video projectors, bearing stereo ormultiple-angles-of-view information. The images can be viewed from theopposite side of the screen at various locations in space. As one movesto various locations, around the screen, the images are viewable, one ata time, without image overlap. This creates several orthoscopic as wellas pseudoscopic viewing zones in space. If one positions his eyes in anorthoscopic viewing zone such that one image goes to each eye, a 3-Dview will be visible. Many viewers will be able to view an orthoscopic3-D video image from several angles and positions at once. This type ofscreen can also be used in front projection with a regular screen behinda lenticular lens.

Another method of preparing stereo visual data for 3-D viewing uses halfwaveplate strips to rotate the plane of polarization 90° for alternatingcolumns of pixels. The columns would be addressed so that every othercolumn would produce a right-eye image and the intervening columns wouldproduce a lefteye image. Alternatively, instead of alternating columns,alternating rows,could be used for the presentation of left and righteye images. Other presentation patterns could be used to present a moreuniform integration of left and right eye images such as having each rowconsist of alternating left and right eye image pixels followed by a rowoffset by one pixel such that a checkerboard pattern of left and righteye pixel images is produced. All pixels corresponding to one eye'simage can be covered with a half waveplate so that one eye's image ispolarized orthogonal to the other eye's image. With this arrangement asingle projector with three or even one light valve can be used toproject onto a non-depolarizing screen for viewing with polarizedglasses and the stereo images will always be in registration withoutrequiring alignment.

In using any of the methods described herein for filling in spacesbetween pixels, the data for each eye's view can be made to overlap thedata for the other eye's view on the screen. This will cause each eye'simage to appear continuous without holes, lines, pixels or other spaces.

Half waveplates may be made pixel-sized and placed over the correctpixels by photo-lithography technology. A photographic mask,corresponding to the pattern of pixels to be viewed by one eye, isimaged with U.V. onto photoresist which is coated onto birefringentplastic of the proper thickness. Once the photoresist is developed awayin the exposed area (or unexposed areas, depending on the resist used),a chemical can be used to dissolve away the plastic that is exposed.Subsequently, the remaining resist is washed away, leaving a mask to beplaced on the light valve. Alternatively, a master dye can be similarlymade of metal which can then be used to punch out holes in theappropriate places in a sheet of birefringent plastic to produce themask for the light valve.

A light valve that is addressed in alternating vertical columns of rightand left eye views can be projected onto a lenticular lens screen (infront or rear projection) to produce an auto-stereoscopic display whichcan be viewed without glasses to produce a 3-D image.

With the use of digital circuits and computer capability built into thesystem, the system can be used to process images so as to turn atwo-dimensional image into a three-dimensional image. One method ofdoing this requires pre-processing of the movie to convert it to 3-D.The conversion need be done only once, with the converted version beingstored for projection at a later time. With this technique, objects in ascene which should appear to the viewer to be located somewhere otherthan in the plane of the screen can be selected during pre-processingand marked. Software can direct a computer to follow the marked objectfrom frame to frame. This allows the operator to select an object onlyonce until it disappears from view, eliminating the need to mark theobject in every frame. Once an object in a scene is selected and markedand the depth at which it is to appear is determined and input, thecomputer can generate a duplicate image of that object at a spacing tothe primary image that will cause the eyes to see the merged image atthe desired depth. Using, for instance, the stereo system, describedabove, in which two projection systems have their images perpendicularlypolarized, to be viewed by someone wearing polarized glasses, thecomputer can generate this duplicated image for projection withpolarization perpendicular to the first image. The projector willproject this duplicate image on the screen next to its counterpartimage, separated by a distance, which determines the depth at which aviewer will see the composite image. When an object is selected tochange its depth inputting this fact and indicating its new depth willcause the computer to change the distance between the two componentimages to be projected on the screen. This will cause the viewer to seethe composite image formed in his brain by binocular fusion at the newdepth.

Another technique can be used to create depth in an image, utilizing theabove-described projection systems. With this technique however,conversion to 3-D occurs as the image is projected with no humanintervention or preprocessing necessary. The imagery however should beshot with this system in mind if the depth created is to be realistic.By having the projector store, for example, three frames at a time andproject, as the stereo frames to be viewed, frames 1 and 4 at any giventime (4 being the current frame being shown, for instance, and 1 beingthe frame which was shown four frames ago), a 3-D view is created usingglasses or an autostereo screen as described herein. The faster anobject moves, the larger the distance will be between the left and righteye images and thus the further behind or in front of the screen theimage will appear to the viewer. Consequently, motion of objects shouldbe coordinated with their depth to provide the most realisticthree-dimensional imagery.

Various recently developed technological innovations such as wirelesstransmission of sound from the projector to speakers, wirelesstransmission of cable and VCR signals to the projector, a built-in VCRand/or a built-in computer when built into a projection system asdescribed herein will produce a projection system with much broader usethan any other system available today.

While the preferred and alternate embodiments of the invention have beenillustrated in detail, modifications and adaptations of such embodimentswill be apparent to those skilled in the art. However, it is to beexpressly understood that such modifications and adaptations are withinthe spirit and scope of the present invention as set forth in thefollowing claims.

We claim:
 1. A display system, comprising:an element for forming animage thereon and having a plurality of pixel holes; a light source forilluminating said image-forming element; means located between saidlight source and said-image-forming element for increasing collimationof light illuminating said image-forming element by increasing adiameter of a light beam radiated from said light source so that thelight beam diameter becomes larger than said image-forming element, saidcollimation increasing means comprising means for folding light, whichotherwise would miss said image-forming element; and at least one inputlens array located between said light folding means and having aplurality of lens elements for focusing the folded light into pixelholes of said image-forming element; wherein said light folding meansfolds the light at angles such that each lens element of said pluralityof lens elements focuses light into a pixel hole which is displacedvertically and horizontally from a pixel hole directly in line with saideach lens element.
 2. A display system, comprising:an element forforming an image thereon and having a plurality of pixel holes; a lightsource for illuminating said image-forming element; means locatedbetween said light source and said-image-forming element for increasingcollimation of light illuminating said image-forming element byincreasing a diameter of a light beam radiated from said light source sothat the light beam diameter becomes larger than said image-formingelement, said collimation increasing means comprising means for foldinglight, which otherwise would miss said image-forming element; and atleast one input lens array located between said light folding means andhaving a plurality of lens elements for focusing the folded light intopixel holes of said image forming element; wherein said light foldingmeans folds the light at angles such that each lens element of saidplurality of lens elements focuses light into a pixel hole which isdisplaced vertically by one pixel and horizontally by one and a halfpixels from a pixel hole directly in line with a said each lens element.3. A display system, comprising:an element for forming an image thereonand having a plurality of pixel holes; a light source for illuminatingsaid image-forming element; means located between said light source andsaid-image-forming element for increasing collimation of lightilluminating said image-forming element by increasing a diameter of alight beam radiated from said light source so that the light beamdiameter becomes larger than said image-forming element, saidcollimation increasing means comprising means for folding light, whichotherwise would miss said image-forming element; and at least one inputlens array located between said light folding means and having aplurality of lens elements for focusing the folded light into pixelholes of said image-forming element; wherein said light folding meansfolds the light at angles such that each lens element of said pluralityof lens elements focuses light into a pixel hole which is displacedhorizontally by two pixels from a pixel hole directly in line with saideach lens element.
 4. A display system, comprising:an element forforming an image thereon and having a plurality of pixel holes; a lightsource for illuminating said image-forming element; means locatedbetween said light source and said-image-forming element for increasingcollimation of light illuminating said image-forming element byincreasing a diameter of a light beam radiated from said light source sothat the light beam diameter becomes larger than said image-formingelement, said collimation increasing means comprising means for foldinglight, which otherwise would miss said image-forming element; and atleast one input lens array located between said light folding means andhaving a plurality of lens elements for focusing the folded light intopixel holes of said image forming element; wherein said light foldingmeans folds the light at angles such that each lens element of saidplurality of lens elements focuses light into a pixel hole which isdisplaced vertically by two pixels from a pixel hole directly in linewith said each lens element.
 5. A display system, comprising:an elementfor forming an image thereon and having a plurality of pixel holes; alight source for illuminating said image-forming element; means locatedbetween said light source and said-image-forming element for increasingcollimation of light illuminating said image-forming element byincreasing a diameter of a light beam radiated from said light source sothat the light beam diameter becomes larger than said image-formingelement, said collimation increasing means comprising means for foldinglight, which otherwise would miss said image-forming element; and atleast one input lens array located between said light folding means andhaving a plurality of lens elements for focusing the folded light intopixel holes of said image forming element; wherein said light foldingmeans folds the light at angles such that each lens element of saidplurality of lens elements focuses light into a pixel hole which isdisplaced vertically by two pixels and horizontally by three pixels froma pixel hole directly in line with a said each lens element.
 6. Adisplay system, comprising:an element for forming an image thereon andhaving a plurality of pixel holes; a light source for illuminating saidimage-forming element; means located between said light source andsaid-image-forming element for increasing collimation of lightilluminating said image-forming element by increasing a diameter of alight beam radiated from said light source so that the light beamdiameter becomes larger than said image-forming element, saidcollimation increasing means comprising means for folding light, whichotherwise would miss said image-forming element; and at least one inputlens array located between said light folding means and having aplurality of lens elements for focusing the folded light into pixelholes of said image forming element; wherein said light folding meansfolds the light at angles such that each lens element of said pluralityof lens elements focuses light into a pixel hole which is displacedvertically by one pixel and horizontally by one-half pixel from a pixelhole directly in line with said each lens element.
 7. A display system,comprising:an element for forming an image thereon and having aplurality of pixel holes; a light source for illuminating saidimage-forming element; means located between said light source and saidimage-forming element for increasing collimation of light illuminatingsaid image-forming element, said collimation increasing means comprisingmeans for utilizing non-well collimated light by redirecting it towardthe image-forming element at one of a multiplicity of acceptance angles;and at least one input lens array located between said collimationincreasing means and said image-forming element for focusing thecollimated light and the redirected non-well collimated light into thepixel holes of said image forming element; wherein said collimationincreasing means increases the collimation of light by increasing adiameter of a light beam radiated from said light source so that thelight beam diameter becomes larger than said image-forming element, saidcollimation increasing means comprising means for folding light whichotherwise would miss said image-forming element, and wherein said atleast one input lens array comprises a plurality of lens elements forfocusing the folded light into pixel holes of said image-formingelement, and wherein said plurality of lens elements comprises aplurality of prisms.
 8. The display system of claim 7, wherein saidcollimation increasing means increases the diameter of the light beam toan extent such that an angle of divergence of the light emerging fromsaid collimation increasing means is close to an acceptable angle ofsaid lens elements.
 9. The display system of claim 8, wherein each lenselement focuses light into a pixel hole other than a pixel hole in linewith a corresponding lens element.
 10. A display system, comprising:anelement for forming an image thereon and having a plurality of pixelholes; a light source for illuminating said image-forming element; meanslocated between said light source and said image-forming element forincreasing collimation of light illuminating said image-forming element,said collimation increasing means comprising means for utilizingnon-well collimated light by redirecting it toward the image-formingelement at one of a multiplicity of acceptance angles; and at least oneinput lens array located between said collimation increasing means andsaid image-forming element for focusing the collimated light and theredirected and non-well collimated light into the pixel holes of saidimage-forming element; wherein said collimation increasing meansincreases the collimation of light by increasing a diameter of a lightbeam radiated from said light source so that the light beam diameterbecomes larger than said image-forming element, said collimationincreasing means comprising means for folding light, which otherwisewould miss said image-forming element, wherein said at least one inputlens array comprises a plurality of lens elements for focusing thefolded light into pixel holes of said image-forming element, and whereinsaid display system further comprises an elliptical reflector forcollecting light from said light source.
 11. The display system of claim10, further comprising means for reflecting light at an angle to an axisof said elliptical reflector before light is directed toward saidimage-forming element.
 12. A display system, comprising:an element forforming an image thereon and having a plurality of pixel holes; a lightsource for illuminating said image-forming element; means locatedbetween said light source and said image-forming element for increasingcollimation of light illuminating said image-forming element, saidcollimation increasing means comprising means for utilizing non-wellcollimated light by redirecting it toward the image-forming element atone of a multiplicity of acceptance angles; and at least one input lensarray located between said collimation increasing means and saidimage-forming element for focusing the collimated light and theredirected, non-well collimated light into the pixel holes of saidimage-forming element; wherein said collimation increasing meansincreases the collimation of light by increasing diameter of a lightbeam radiated from said light source so that the light beam diameterbecomes larger than said image-forming element, said collimationincreasing means comprising means for folding light which otherwisewould miss said image-forming element, wherein said at least one inputlens array comprises a plurality of lens elements for focusing thefolded light into pixel holes of said image-forming element, and whereinsaid display system further comprises at least one additional lightsource for illuminating said image-forming element.
 13. The displaysystem of claim 12, further comprising a collimating lens locatedadjacent to said image-forming element.
 14. A display system,comprising:an element for forming an image thereon and having aplurality of pixel holes; a light source for illuminating saidimage-forming element; means located between said light source and saidimage-forming element for increasing collimation of light illuminatingsaid image-forming element, said collimation increasing means comprisingmeans for utilizing non-well collimated light by redirecting it towardthe image-forming element at one of a multiplicity of acceptance angles;and at least one input lens array located between said collimationincreasing means and said image-forming element for focusing thecollimating light and the redirected non-well collimated light into thepixel holes of said image-forming element; wherein said collimationincreasing means increases the collimation of light by increasing adiameter of a light beam radiated from said light source so that thelight beam diameter becomes larger than said image-forming element, saidcollimation increasing means comprising means for folding light whichotherwise would miss said image-forming element, wherein said at leastone input lens array comprises a plurality of lens elements for focusingthe folded light into pixel holes of said image-forming element, andwherein said display system further comprises light deflectors locatedbetween said light source and said image-forming element for deflectinglight that misses said image forming element for passing through thepixel holes therein.
 15. The display system of claim 14, wherein saidlight deflectors surround a light beam directly illuminating saidimage-forming element.